WASHINGTON — Attorney General William P. Barr on Friday labeled the images of the death of George Floyd, a black man in Minneapolis whom a white police officer knelt on for nearly nine minutes, as “harrowing” and “deeply disturbing” and vowed that the federal investigation into his death would proceed quickly.

“I am confident justice will be served,” Mr. Barr said in a statement as protesters across the country condemned the actions of the officer, Derek Chauvin, who was charged Friday by the local authorities with third-degree murder and manslaughter.

But the Trump administration’s years of inaction on police violence and President Trump’s embrace of law enforcement have made civil rights advocates wary of the Justice Department’s involvement in the Floyd case. The administration has largely dismantled police oversight efforts, curbing the use of federal consent decrees to overhaul local police departments. Mr. Barr has said that communities that criticize law enforcement may not deserve police protection, and Mr. Trump has encouraged officers not to be “too nice” in handling suspects.

Advocates for police overhaul said in interviews on Friday that they were in a difficult position: After denouncing the federal government’s retreat from police accountability — and civil rights enforcement more broadly — they are now wary of its intentions.

“Our confidence in a federal intervention in cases like this is wholly dependent on the track record of the administration that is stepping in,” said Derrick Johnson, the president of the N.A.A.C.P. “This administration lacks credibility when it comes to addressing issues of justice, fairness and race.”

Mr. Barr said that any federal charges would be “based on the law and facts,” and that they would not come until after local charging decisions.

To bring federal civil rights charges in the Floyd case, prosecutors have to meet a difficult bar: proving that Mr. Chauvin intended to violate Mr. Floyd’s civil rights and acted on that wish, Mr. Johnson said. Prosecutors are often reluctant to bring such cases because they are so difficult to win.

“We are confronted with the stark reality this family may not see justice, even with the prevailing evidence in broad daylight from multiple camera views that there was no resisting, no physical provocation, that he was subdued and cuffed, that he said he can’t breathe, and that blood was coming out of his mouth,” Mr. Johnson said.

Mr. Barr’s announcement suggested no broader investigation into possible abuses in the Minneapolis Police Department, a move that local activists have demanded. Congressional Democrats also asked the Justice Department this week to open an investigation into the police. The city has a history of accusations of police abuse, and in 2017, an officer in a Minneapolis suburb was found not guilty of manslaughter in the death of Philando Castile, a black motorist.

Under President Barack Obama, the Justice Department aggressively sought to combat excessive use of force by the police. The department and local police departments signed 14 consent decrees, court-enforced agreements detailing remedies like additional police training or data collection.

The Justice Department under Mr. Obama most likely “would have looked at Minneapolis given the pattern of problems that were apparent,” said Jonathan Smith, a former department official who negotiated several of the decrees.

Justice Department findings typically prompted the consent agreements. Police in Ferguson, Mo., where the fatal shooting in 2014 of an unarmed black teenager by a white officer set off a national debate over the use of police force, fined and arrested African-Americans in part to balance the city’s budget, the department found. It concluded that the Baltimore police were more concerned with accumulating statistics than reducing violent crime.

Mr. Trump’s first attorney general, Jeff Sessions, moved to rein Justice Department efforts to investigate patterns of allegations of misconduct by the local police.

The department under Mr. Trump has entered into consent decrees with the police in Ville Platte and the sheriff’s office in Evangeline Parish, La., based on an investigation opened in 2015, and opened a so-called pattern-and-practice investigation into the police in Springfield, Mass. It has also pursued abuses in prison systems in Alabama and New Jersey.

But critics said that the department’s overall approach did little to address allegations of police misconduct.

“They’ve just really been dismissive about the fact that police abuse happens, and the fact that it’s a problem the federal government can and should do something about,” said Christy E. Lopez, a former deputy chief in the special litigation section of the civil rights division during the Obama administration.

She pointed to comments that Mr. Trump made in 2017 to the police on Long Island, suggesting that they should not protect the heads of suspects ushered into police cars. At the time, police departments around the country distanced themselves from the president’s position.

“It’s been the tenor of this entire administration and of the D.O.J.,” Ms. Lopez said. “That absolutely sends a message to police officers on the street.”

Mr. Sessions repeatedly argued that criticism of the police or excessive oversight could damage the morale of officers, harming their ability to control crime. Mr. Barr, in his own public comments, has stressed that abuses reflect “bad apples” more than systemic breakdowns.

Minority communities have long criticized Mr. Barr’s full-throated support of law enforcement, beginning with his first stint as attorney general under President George Bush, when he advocated on behalf of maximum sentencing laws that laid the groundwork for high rates of incarceration among black people.

Civil rights groups, including the N.A.A.C.P., opposed his confirmation last year. During his confirmation hearing, Mr. Barr defended his record but said that such a strict approach on sentencing was now unnecessary.

Critics wary of the Justice Department’s intervention in the Floyd case also pointed to statements Mr. Barr made over the past year in which he backed the police and cast protesters as endangering public safety.

He warned in December that critics of the police risked losing law enforcement protection, saying, “If communities don’t give that support and respect, they may find themselves without the police protection they need.” And he said last summer that officers were unfairly scrutinized when people resisted arrest while the fact that suspects who resisted arrest, endangering themselves and the police, “frequently goes without mention.”

“The cynicism that people have is well-grounded,” said David Rudovsky, a civil rights lawyer in Philadelphia and a senior fellow at the University of Pennsylvania Law School who has long worked on police accountability cases.

The burden in the Floyd case is on law enforcement “to show that they can actually do a credible investigation,” he said.

Mr. Barr’s defenders pointed to his first stint as attorney general, when the acquittals of four white Los Angeles police officers in the beating of a black motorist, Rodney King, incited protests and riots in Los Angeles and across the country.

The Justice Department intervened, charging the officers with violating Mr. King’s civil rights. Federal prosecutors secured guilty verdicts against two of the officers, and many civil rights advocates said that the federal government had helped secure justice for Mr. King.

“Bill did take a very personal interest in the case,” said George Terwilliger, Mr. Barr’s top deputy at the time. “The damage that can be done to law enforcement interests when officers engage in criminal acts can’t be underestimated.”

Mr. Terwilliger said that every attorney general should have a record of solid support for police, “but that doesn’t mean that when law enforcement authority is abused, that you’re going to lay down.”

Mr. Floyd’s death has echoes of the death of Eric Garner, who died after an officer on Staten Island wrapped his arm around Mr. Garner’s neck. The encounter was captured on video and Mr. Garner’s dying words of “I can’t breathe” became a rallying cry for demonstrations.

A local grand jury voted not to indict the officer, Daniel Pantaleo, and the Justice Department declined to charge him after a five-year investigation, saying it could not prove beyond a reasonable doubt that he had willingly and knowingly violated Mr. Garner’s civil rights. The New York Police Department ultimately dismissed Mr. Pantaleo over his use of an illegal chokehold on Mr. Garner.

Fired officer is charged with third-degree murder after George Floyd’s death.

The former police officer who was seen on video using his knee to pin down George Floyd, a black man who later died, has been arrested and charged with murder, the authorities announced on Friday, after days of growing unrest in Minneapolis escalated with the burning of a police station and protests that drew attention from the White House.

The former Minneapolis police officer, Derek Chauvin, who is white, was arrested by the Minnesota Bureau of Criminal Apprehension on Friday, the authorities said. Mr. Chauvin, 44, was charged with third-degree murder and second-degree manslaughter, Mike Freeman, the Hennepin County attorney, announced on Friday afternoon.

Mr. Floyd’s relatives said in a statement that they were disappointed by the decision not to seek first-degree murder charges.

Third-degree murder does not require an intent to kill, according to the Minnesota statute, only that the perpetrator caused someone’s death in a dangerous act “without regard for human life.” Charges of first- and second-degree murder require prosecutors to prove, in almost all cases, that the perpetrator made a decision to kill the victim.

Mr. Chauvin was also charged with second-degree manslaughter, a charge that requires prosecutors to prove he was so negligent as to create an “unreasonable risk,” and consciously took the chance that his actions would cause Mr. Floyd to be severely harmed or die.

An investigation into the other three officers who were present at the scene on Monday was ongoing, Mr. Freeman said.

The developments came after a night of chaos in which protesters set fire to a police station in Minneapolis, the National Guard was deployed to help restore order, and President Trump injected himself into the mix with tweets that appeared to threaten violence against protesters.

The tensions in Minneapolis reflected a growing frustration around the country, as demonstrators took to the streets to protest the death of Mr. Floyd and other recent killings of black men and women.

Mr. Floyd, 46, died on Monday after pleading “I can’t breathe” while Mr. Chauvin pressed his knee into Mr. Floyd’s neck, in an encounter that was captured on video.

Gov. Tim Walz of Minnesota, a Democrat, expressed solidarity with the protesters during a news conference on Friday, but said that a return to order was needed to lift up the voices of “those who are expressing rage and anger and those who are demanding justice” and “not those who throw firebombs.”

President Trump, who previously called the video of Mr. Floyd’s death “shocking,” drew criticism for a tweet early Friday that called the protesters “thugs” and said that “when the looting starts, the shooting starts.” The comments prompted Twitter to attach a warning to the tweet, saying that it violated the company’s rules about “glorifying violence.”

The president gave his first extensive remarks on the protests later on Friday at the White House, declaring that “we can’t allow a situation like happened in Minneapolis to descend further into lawless anarchy and chaos. It’s very important, I believe, to the family, to everybody, that the memory of George Floyd be a perfect memory.”

Addressing his earlier Twitter comments, Mr. Trump said, “The looters should not be allowed to drown out the voices of so many peaceful protesters. They hurt so badly what is happening.”

The spectacle of a police station in flames and a president appearing to threaten violence against those protesting the death of a black man in police custody, set against the backdrop of a coronavirus pandemic that has kept many people from engaging with one another directly for months, added to the anxiety of a nation already plagued by crises.

The protests — some peaceful, some marked by violence — have spread across the country, from Denver and Phoenix to Louisville, Ky., and Columbus, Ohio, with more expected on Friday night.

Mayor Jacob Frey of Minneapolis imposed an 8 p.m. curfew to try to stem the escalating violence that has engulfed the city for the last three nights.

The curfew will extend through the weekend, according to the mayor’s order, expiring at 6 a.m. each morning. During the hours of the curfew, people are prohibited from traveling on public streets or gathering in a public place.

But even as the curfew was taking effect on Friday evening, protesters were defying it, gathering in the streets around the police station that was burned a night earlier.

They chanted, “No justice, no peace, prosecute the police!”

Law enforcement officials fired tear gas into the streets and patrolled in military vehicles.

Governor Walz, who activated the National Guard on Thursday as local police appeared to lose control over angry demonstrators, also extended the curfew to St. Paul and said guardsmen would return to the streets in anticipation of more protests.

In the unrest on Thursday night, more than 160 buildings were destroyed, damaged or looted, The Star Tribune reported. Nearly all businesses in the Seward neighborhood of Minneapolis were shut on Friday, many protected with plywood.

During a 90-minute news conference on Friday, the governor said officials should have anticipated that the protests could become violent, but he said it was unrealistic to expect law enforcement to stop people from coming out to demonstrate, even amid the social-distancing orders that have been imposed during the coronavirus pandemic.

“Watching what happened to George Floyd had people say, ‘To hell with staying home,’” he said. “The idea that we would go in and break up those expressions of grief and rage was ridiculous.”

Camille J. Gage, 63, an artist and musician who joined the protests, said she was relieved that Mr. Chauvin had been charged. “How can anyone watch that video and think it was anything less?” she said. “Such blatant disregard for another living soul.”

Kelsey Lindell, 27, an executive producer for a local film company, said all four officers at the scene of the incident should be arrested, charged and punished for murder. “I want to see a higher charge for all the officers,” she said, “but the biggest thing for me is that this guy gets jail time.”

Mr. Walz acknowledged that the Minneapolis police had lost the trust of city residents, but he implored residents to see the National Guard as a peacekeeping force meant to keep “anarchists” from taking over and destroying more of the city.

“I need to ask Minnesotans, those in pain and those who feel like justice has not been served yet, you need to help us create the space so that justice will be served,” the governor said. “It is my expectation that it will be swift.”

Days of protests had intensified on Thursday night when the Minneapolis Police Department’s Third Precinct station house was overrun by a crowd of protesters, with some people tossing fireworks and other items at officers, while the police fired projectiles back.

Officers retreated in vehicles just after 10 p.m. Thursday local time as protesters stormed the building — smashing equipment, lighting fires and setting off fireworks, according to videos posted from the scene.

Mr. Frey said at a news conference Friday morning that he had made the call for officers to flee the Third Precinct, saying, “The symbolism of a building cannot outweigh the importance of life.”

Mr. Frey, a Democrat, said he understood the anger of the city’s residents but pleaded with people to stop destroying property and looting stores. “It’s not just enough to do the right thing yourself,” he said. “We need to be making sure that all of us are held accountable.”John Harrington, the commissioner of the Minnesota Department of Public Safety, said that arrests had been made related to looting on Thursday night, but that he did not know how many. The arrests included people breaking into the grocery stores, Targets and pharmacies, he said.

A demonstration turned destructive in Atlanta on Friday night, as hundreds of protesters took to the streets, smashing windows and clashing with the police.

They gathered around Centennial Olympic Park, the city’s iconic tourist destination. People jumped on police cars. Some climbed atop a large red CNN sign outside the media company’s headquarters and spray-painted messages on it. Some threw rocks at the glass doors of the Omni Hotel, eventually breaking the glass. Others shattered windows at the College Football Hall of Fame, where people rushed in and emerged with branded fan gear.

Jay Clay, 19, an Atlanta resident and graphic designer, watched the protests from across a street with a mixture of curiosity and solidarity.

“After all this injustice and prejudice, people get fed up,” Mr. Clay said. “I wanted to come down and check it out. But this feels like it’s getting out of hand.”

Mayor Keisha Lance Bottoms pleaded for calm as the demonstrations unfolded.

“It’s enough. You need to go home,” she said. “We are all angry. This hurts. This hurts everybody in this room. But what are you changing by tearing up a city? You’ve lost all credibility now. This is not how we change America. This is not how we change the world.”

Bernice King, the youngest daughter of the Rev. Dr. Martin Luther King Jr., also spoke at the news conference, invoking her father’s legacy.

“Violence in fact creates more problems. It is not a solution,” Ms. King said. She said she felt and understood the anger of protesters but added, “There are people who would try to incite a race war in this country. Let’s not fall into their hands and into their trap. There’s another way.”

As the protests went on, police officers in riot gear were gathering. By 9:30 p.m., tear gas canisters were launched, and a wave of protesters ran back toward the park.

Chanting “Hands up! Don’t shoot,” and “I can’t breathe,” thousands of protesters gathered in cities across the country on Friday, the third consecutive day of mass demonstrations after Mr. Floyd’s death.

In Washington, a large crowd gathered and chanted outside the White House, prompting the Secret Service to temporarily lock down the building. Video on social media showed demonstrators knocking down barricades, confronting police officers and spray painting other buildings nearby.

In Denver, hundreds of protesters converged on Civic Center Park, waving signs and chanting as Sam Cooke’s “A Change Is Gonna Come” played over a loudspeaker. Some thrust fists in the air and scrawled messages on the ground in chalk, according to a live broadcast of the event by ABC News.

Bus and rail service into and out of downtown Denver was suspended in response to the protest Friday and another one planned for Saturday, which is expected to draw more than 10,000 people, according to the Regional Transportation District in Denver.

The agency said it had suspended the service “to ensure the safety of our staff and our riders.”

In Houston, a march in downtown Houston briefly turned chaotic as the windows of a police SUV were smashed and protesters were arrested. At one point, officers holding batons encircled a downtown street as other officers arrested a demonstrator who was on the ground.

In Milwaukee, protesters shouted “I can’t breathe” — echoing Mr. Floyd’s anguished plea and the words of Eric Garner, a black man who died in New York Police Department custody in 2014. The protesters briefly shut down part of Interstate 43, a major highway in Milwaukee, according to WTMJ-TV. A Milwaukee police spokeswoman referred questions to the county sheriff’s office, which did not immediately respond to messages.

In Detroit, a small crowd gathered outside police headquarters, declaring, “Black is not a crime.”

Leading the crowd in a chant, Mary Sheffield, a member of the Detroit City Council, proclaimed, “I’m fired up. I’m fed up.” She said what she saw on her television and how the country values black lives is unacceptable.

“To be honest with you, I’m tired of rallying. I’m tired of marching. I’m tired of asking. I’m tired of arguing. I’m just tired,” she said. “We must come to a point where we are demanding now. We are demanding justice.”

Later, the demonstration in Detroit swelled to more than 1,000, as protesters marched on major thoroughfares leading downtown, blocking traffic along the way.

In San Jose, Calif., protesters marched through that city’s downtown before blocking Highway 101, standing across at least five lanes of traffic, according to NBC News Bay Area.

Tensions flared in New York for the second night in a row as thousands of protesters stormed the perimeter of Barclays Center in Brooklyn, trading projectiles of plastic water bottles, debris and tear gas and mace with police officers.

The protest had begun peacefully Friday afternoon, with hundreds chanting, “Black lives matter,” and, “We want justice,” in downtown Manhattan. But the demonstrations took a turn in Brooklyn.

Police officers with twist-tie handcuffs hanging from their belts stood next to Department of Corrections buses and squad cars with lights flashing, encircling the perimeter. A police helicopter and a large drone whirred in the hot air overhead.

Protesters were later seen throwing water bottles, an umbrella and other objects at officers, who responded by shooting tear gas into the crowd.

Earlier in the evening, several hundred people filled Foley Square near the city’s criminal courthouses. After a man in a green sweatshirt crossed a police barricade, he was swarmed by officers while protesters screamed. He was led away on foot in handcuffs.

“It was kind of his mistake,” said Jason Phillips, 27, of Queens. “But they were trying to push him back, and as they pushed him back, he slipped, and they took that as some type of threat.”

Despite the frustrations of demonstrators on Friday, the police said the number of people detained was much smaller than the night before, when 72 people were arrested.

In a probable cause affidavit released on Friday after the charges against Mr. Chauvin were filed, prosecutors said that the former officer held his knee to Mr. Floyd’s neck for more than eight minutes. “Two minutes and 53 seconds of this was after Mr. Floyd was non-responsive,” the affidavit said.

But preliminary results from an autopsy indicated that Mr. Floyd did not die from suffocation or strangulation, prosecutors wrote, and that “the combined effects” of an underlying heart condition, any potential intoxicants and the police restraint likely contributed to his death. He also began complaining that he could not breathe before he was pinned down, the affidavit said.

The officers’ body cameras were running throughout the encounter, prosecutors said.

Four officers responded to a report at about 8 p.m. on Monday about a man suspected of making a purchase from a store with a fake $20 bill, prosecutors said. After learning that the man was parked near the store, the first two responding officers, who did not include Mr. Chauvin, approached Mr. Floyd, a former high school sports star who worked as a bouncer at a restaurant in Minneapolis.

Mr. Floyd, who was in a car with two other people, was ordered out and arrested. But when the officers began to move him toward a squad car, he stiffened and resisted, according to the affidavit. While still standing, Mr. Floyd began to say he could not breathe, the affidavit said.

That was when Mr. Chauvin, who was among two other officers who arrived at the scene, got involved, prosecutors said. Around 8:19 p.m., Mr. Chauvin pulled Mr. Floyd out of the squad car and placed his knee onto Mr. Floyd’s neck area, holding him down on the ground while another officer held his legs. At times, Mr. Floyd pleaded, the affidavit said, saying, “I can’t breathe,” “please” and “mama.”

“You are talking fine,” the officers said, according to the affidavit, as Mr. Floyd wrestled on the ground.

At 8:24 p.m., Mr. Floyd went still, prosecutors said. A minute later, one of the other officers checked his wrist for a pulse but could not find one. Mr. Chauvin continued to hold his knee down on Mr. Floyd’s neck until 8:27, according to the affidavit.

The other officers, who have been identified as Thomas Lane, Tou Thao and J. Alexander Kueng, are under investigation. Mr. Freeman, the county attorney, said he expected to bring more charges in the case but offered no further details.

Ben Crump, a civil rights lawyer representing Mr. Floyd’s family, released a statement on Friday calling the arrest of Mr. Chauvin “a welcome but overdue step on the road to justice.” But he said the charges did not go far enough.

“We expected a first-degree murder charge. We want a first-degree murder charge. And we want to see the other officers arrested,” said the statement, which was attributed to Mr. Floyd’s family and to Mr. Crump.

“The pain that the black community feels over this murder and what it reflects about the treatment of black people in America is raw and is spilling out onto streets across America,” the statement said.

Mr. Crump and the family said they want Minneapolis — and other cities across the country — to fix deficiencies in policies and training that they said permitted Mr. Floyd’s death and others like it.

Among the areas they said they want addressed are the use of appropriate, nonlethal restraint techniques, the ability to recognize the medical signs associated with the restriction of airflow, and the legal duty to seek emergency medical care and stop a civil rights violation.

“For four officers to inflict this kind of unnecessary, lethal force — or watch it happen — despite outcry from witnesses who were recording the violence — demonstrates a breakdown in training and policy by the city,” the statement said. “We fully expect to see the other officers who did nothing to protect the life of George Floyd to be arrested and charged soon.”

Mr. Floyd’s family is being forced “to explain to his children why their father was executed by police on video,” they said.

Richard Frase, a professor of criminal law at the University of Minnesota, said it was reasonable for prosecutors to charge Mr. Chauvin with third-degree murder, as opposed to a more severe form of murder, which would require proving that Mr. Chauvin intended to kill Mr. Floyd.

Professor Frase said the case against Mr. Chauvin appeared to be even stronger than the one that Hennepin County prosecutors brought against Mohamed Noor, a former Minneapolis police officer who shot and killed Justine Ruszczyk in 2017.

Mr. Noor was charged with the same combination of crimes, third-degree murder and second-degree manslaughter, and was convicted of both.

In that case, Professor Frase said, the officer had seemingly panicked and fired a single shot. “There’s a question of whether he even had time to be reckless,” he said, referring to Mr. Noor. “Here, there’s eight minutes.”

The criminal complaint against Mr. Chauvin, Professor Frase said, did not identify any specific motive for officers to kill Mr. Floyd, which he said essentially ruled out first or second-degree murder unless additional evidence surfaced.

Professor Frase said he expected Mr. Chauvin’s lawyers to seize on the preliminary autopsy findings that showed that Mr. Floyd had not died of asphyxiation, which could form the basis for an argument that there was no way Mr. Chauvin could have expected him to die. But Professor Frase said another common strategy used by police officers facing charges of brutality — arguing that they were in harm’s way — may be unlikely to convince a jury.

“In this case, there was nobody but Mr. Floyd in danger,” he said. “And there was all that time when it seems there was no need to keep kneeling on his neck like that.”

A government drone in the skies over Minneapolis stokes civil liberties concerns.

A Predator drone operated by the federal Customs and Border Protection agency flew a surveillance mission over Minneapolis on Friday morning as the city reeled from days of escalating violence, stoking suspicion and prompting criticism from civil liberties groups.

An agency spokesman said in a statement that the unmanned aircraft “was preparing to provide live video to aid in situational awareness at the request of our federal law enforcement partners in Minneapolis.”

But after more than an hour flying in a holding pattern at 20,000 feet over the city, according to publicly available flight data, the drone returned to its base in North Dakota. “The requesting agency determined that the aircraft was no longer needed,” the statement said.

In recent years, U.S. government agencies have used surveillance aircraft to monitor protests in American cities. The American Civil Liberties Union, which is suing the Baltimore Police Department to block its surveillance plane program, called on Friday for Customs and Border Protection to immediately halt the use of its drone over Minneapolis.

“This rogue agency’s use of military technology to surveil protesters inside U.S. borders is deeply disturbing,” Neema Singh Guliani, a lawyer for the group, said in a statement.

The tweet from President Trump suggesting that protesters in Minneapolis could be shot violated Twitter’s rules against “glorifying violence,” the company said on Friday, escalating tensions between the president and his favorite social media megaphone and injecting Mr. Trump into a growing crisis over police abuse and race that will be another test of his ability to lead an anxious nation.

The company prevented users from viewing Mr. Trump’s message, which contained the phrase, “when the looting starts, the shooting starts,” without first reading a brief notice describing the rule violation and also blocked users from liking or replying to Mr. Trump’s post. But the site did not take the message down, saying it was in the public interest for the president’s words to remain accessible.

Mr. Trump attempted to explain his earlier tweets in new postings on Friday afternoon. “Looting leads to shooting,” he said, pointing to incidents in Minneapolis and Louisville, Ky., during protests in both places this week. “I don’t want this to happen, and that’s what the expression put out last night means.”

When a reporter at the White House later asked whether Mr. Trump was aware of the racist history of the phrase he had used, Mr. Trump said he had heard it for years, but said he was not aware that it had been used by Walter E. Headley, a former Miami police chief, during a news conference in December 1967. The chief’s comment further inflamed racial tensions in that city, and riots broke out the following year.

“When there’s looting,” Mr. Trump said, explaining the intentions behind his tweet, “people get shot and they die.”

Mr. Trump also said that he had spoken to members of Mr. Floyd’s family, calling them “terrific people.”

Mr. Trump had begun tweeting about the unrest in Minneapolis around 1 a.m., as cable news showed a Minneapolis police station engulfed in a fire set by protesters. He called the protesters “thugs.”

Chief Headley attracted national attention in the late 1960s for using shotguns, dogs and other heavy-handed policies to fight crime in the city’s black neighborhoods. “We haven’t had any serious problems with civil uprising and looting, because I’ve let the word filter down that when the looting starts, the shooting starts,” he said in 1967, adding, “we don’t mind being accused of police brutality.”

When asked about Mr. Trump’s tweet on Friday, Governor Walz said, “It’s just not helpful.” “Anything we do to add fuel to that fire is really, really challenging,” he added.

Obama and Biden addressed Mr. Floyd’s death.

Former President Barack Obama on Friday called on the nation to work together to create a “new normal” in which bigotry no longer infects institutions, while former Vice President Joseph R. Biden Jr. used a short speech to call for “justice for George Floyd.”

In a statement posted to Twitter, Mr. Obama said, “It’s natural to wish for life ‘to just get back to normal’ as a pandemic and economic crisis upend everything around us.” But for millions of Americans, being treated differently because of race is “normal,” Mr. Obama said, referencing two other recent cases: Ahmaud Arbery, who was killed after two men confronted him while he was running in South Georgia, and Christian Cooper, who was bird watching in Central Park when a woman called police to say she was being threatened.

“This shouldn’t be ‘normal’ in 2020 America,” Mr. Obama said, adding,

“It falls on all of us, regardless of our race or station, to work together to create a ‘new normal’ in which the legacy of bigotry and unequal treatment no longer infects our institutions or our hearts.”

Mr. Biden, the presumptive Democratic presidential nominee, rebuked President Trump for his response to the protests in Minneapolis.

“This is no time for incendiary tweets,” Mr. Biden said in a brief speech delivered via livestream. “It’s no time to encourage violence. This is a national crisis, and we need real leadership right now. Leadership that will bring everyone to the table so we can take measures to root out systemic racism.” He did not mention Mr. Trump by name.

Describing the United States as “a country with an open wound,” Mr. Biden called for “real police reform” so that “bad cops” are held accountable.

Mr. Biden said he had just spoken with members of Mr. Floyd’s family, and he addressed them as he concluded his speech. “I promise you, I promise you, we’ll do everything in our power to see to it that justice is had in your brother, your cousin’s case,” he said.

Gov. Andrew M. Cuomo of New York, who has risen on the national political stage for his coronavirus response, spoke up in defense of the protesters in Minnesota.

“I stand figuratively with the protesters,” he said on Friday. “I stand against the arson and the burglary and the criminality and I think all well-meaning Americans stand with the protesters. Enough is enough.”

Reporting was contributed by Victoria Bekiempis, Katie Benner, Nicholas Bogel-Burroughs, Audra D.S. Burch, Maria Cramer, Julie Davis, Sopan Deb, Richard Fausset, Thomas Fuller, Katie Glueck, Russell Goldman, John Eligon, Matt Furber, Maggie Haberman, Christine Hauser, Jack Healy, Thomas Kaplan, Michael Levenson, Dan Levin, Neil MacFarquhar, Eric Melzer, Sarah Mervosh, Elian Peltier, Katie Rogers, Edgar Sandoval, Marc Santora, Derrick Bryson Taylor, Neil Vigdor, Mike Wolgelenter and Raymond Zhong.

Top lawmakers in both parties, spurred to action by the death of a black Minnesota man in the custody of white police officers, said on Friday that they would hold hearings in the coming weeks on the use of excessive force by law enforcement and on racial violence.

Representative Jerrold Nadler, Democrat of New York and the chairman of the Judiciary Committee, said in an interview that he would convene a hearing in June to consider new federal actions that could help stem racial violence, especially acts of brutality by law enforcement against black and brown Americans. He also said his committee was looking at a federal chokehold ban and legislation to establish a commission to study the social status of black men and boys.

“What we are going to look at very specifically is where and under what circumstances the federal government can step in when local governments are engaging in or not stopping or controlling violence against racial minorities,” Mr. Nadler said.

In the Senate, Senator Lindsey Graham, Republican of South Carolina and the chairman of that chamber’s Judiciary Committee, said he would seek testimony on proposals to improve policing, combat “racial discrimination regarding the use of force” and improve relations between police departments and the communities they serve. He described the death of George Floyd, a Minnesota man who died after a white police officer knelt on his neck in an episode captured on video, as “horrific.”

Mr. Graham said both he and Senator Dianne Feinstein of California, the top Democrat on the panel, were “appalled at what we saw and believe it is important to have a hearing as soon as possible as to how to combat this outrage.”

“We intend to shine a bright light on the problems associated with Mr. Floyd’s death,” he said in a statement, “with the goal of finding a better way forward for our nation.”

The swift response on both sides of the Capitol reflected the interest by lawmakers in both parties to respond to a rash of killings of black Americans by white civilians and police officers this year, which have inspired protests across the country. But Democrats appeared to be moving more aggressively to advance specific legislative proposals aimed at forcing the Trump administration to mount a stronger response.

Senator Mitch McConnell, Republican of Kentucky and the majority leader, said Friday that the officers involved in the Minneapolis case “look pretty darn guilty,” and he called the death of Mr. Floyd “a hideous crime.” But he condemned violent protests across the country, and said that Americans should trust that the criminal justice system would hold those involved accountable.

“I can speak for myself, I think what’s happening in Louisville and in Minneapolis really needs to stop,” Mr. McConnell said. “This senseless violence and reaction to this is not helpful. But you can certainly understand the outrage — you can certainly understand the outrage.”

Mr. Nadler’s comments came a day after he and the 23 other Democrats on the Judiciary Committee sent a letter to the Justice Department calling on officials there to open investigations related to the recent deaths of Mr. Floyd, Breonna Taylor in Kentucky and Ahmaud Arbery in Georgia.

“In terms of maintaining social peace in this country, it is imperative that we move ahead quickly,” Mr. Nadler said.

Many of the proposals being considered by Democrats have been discussed for years with little movement. Watching the growing national outcry in recent days, Speaker Nancy Pelosi said House Democrats would pursue “action so that this stops,” rather than just offering “expressions of concern.”

“We saw a murder take place before our very eyes,” she said on Thursday. “And so, the fact that the police officers were fired, that’s one thing, but there has to be some justice in all of this.”

In addition to drafting new legislation, Mr. Nadler said he was looking at a bill introduced by Representative Hakeem Jeffries of New York that would place a federal ban on the use of chokeholds or any pressure to the neck by the police.

Mr. Jeffries, who represents Brooklyn, initially introduced the legislation in 2015 after the death of Eric Garner by police chokehold, and it would have made the tactic used by the officer in Mr. Floyd’s case illegal. In videos of the episodes, Mr. Garner and Mr. Floyd could both be heard saying “I can’t breathe” in the moments before their deaths.

The committee is also likely to take up the bill establishing a commission that would be tasked with investigating racial disparities in education, criminal justice, health and employment and recommending policy changes. Sponsored by Representative Frederica Wilson, Democrat of Florida, the bill has also been put forward in the Senate by Senators Marco Rubio, Republican of Florida; Kamala Harris, Democrat of California; and Cory Booker, Democrat of New Jersey.

Representative Karen Bass, Democrat of California and the chairwoman of the Congressional Black Caucus, said in an interview on Friday that she was still looking to develop other proposals to address state laws that protect officers who use force when they say they feel threatened.

“This stuff is happening so fast,” she said, “we literally have not had a chance to have that conversation.”

Ms. Bass is among a group of prominent progressive lawmakers who are pressing for a vote on a separate, nonbinding resolution they introduced on Friday “condemning all acts of police brutality, racial profiling, and the use of excessive and militarized force throughout the country.” It calls on law enforcement agencies at the federal, state and local levels to commit to reforms.

“There can be no justice for George Floyd, Breonna Taylor, or any of the human beings who have been killed by law enforcement,” Representative Ayanna Pressley, Democrat of Massachusetts and a sponsor of the measure, wrote on Twitter. “For in a just world, they would still be alive. There must, however, be accountability.”