Capitol Police arrested a man with an ‘unauthorized’ inauguration credential and a gun at a security checkpoint.

The U.S. Capitol Police arrested a man at a security checkpoint in Washington on Friday after he flashed an “unauthorized” inauguration credential and a search of his truck found an unregistered handgun and more than 500 rounds of ammunition, the authorities said.

The arrested man, Wesley A. Beeler, 31, was a contractor, and the credential was not fake, according to a federal law enforcement official who spoke on the condition of anonymity because he was not authorized to discuss the arrest. A spokeswoman for the Capitol Police said the man had shown a “nongovernment issued” credential of some kind.

Mr. Beeler’s father, Paul Beeler, said in an interview that his son was part of a security team working alongside the Capitol Police and the National Guard, and that his son must have simply left his personal gun in his truck. Wesley Beeler has an active private security license in Virginia and was approved to have a handgun, shotgun or patrol rifle while on assignments, according to a state website.

The arrest comes as law enforcement officials have tried to fortify Washington ahead of Inauguration Day on Wednesday, when they fear that extremists emboldened by the attack on the Capitol by President Trump’s supporters on Jan. 6 could seek to cause violence. A militarized “green zone” is being established downtown, National Guard members are flooding the city, and a metal fence has gone up around the Capitol grounds in advance of the swearing-in of President-elect Joseph R. Biden Jr.

Mr. Beeler, of Front Royal, Va., had driven up to a security checkpoint less than half a mile from the Capitol grounds on Friday evening and presented “an unauthorized inauguration credential,” according to a statement from a Capitol Police officer filed in a District of Columbia court on Saturday. The officer, Roger Dupont, said that he had checked the credential against a list and that he found the man’s credential did not give him the authority to enter the restricted area.

Officers searched the truck, which had several gun-related bumper stickers, and found a loaded Glock pistol, 509 rounds for the pistol and 21 shotgun shells, the police said. Mr. Beeler had admitted having the Glock in the truck’s center console when he was asked if there were weapons in the car, they said.

Mr. Beeler, who could not be reached for comment, was charged with five crimes, including possessing a weapon and ammunition in Washington without having it registered as required. The documents filed in court and an incident report from the city’s Metropolitan Police Department do not shed light on why the man had tried to access a restricted area, nor do they provide more details about the credential the police say he presented.

Paul Beeler said his son, a father of four, had recently been working as a security guard on the Capitol grounds, and that he had held other security jobs over the years. “He was proud of the work he was doing with the police and the National Guard,” the father said.

Asked if he thought his son supported a peaceful transition of power, Paul Beeler said, “That’s the reason he’s there.”

The elder Beeler said he had grown worried about his son when he did not return his text messages on Friday night, and that he had called him on Saturday morning, when he thought his son would be returning to Virginia after his shift. He and his wife discovered that Mr. Beeler had been arrested when she received a call from a reporter, he said.

Law enforcement officials have said they are alarmed by chatter among far-right groups and other racist extremists who are threatening to target the nation’s capital to protest Mr. Biden’s electoral victory. Federal agencies have tried to keep some people who breached the Capitol with weapons earlier this month from returning to the city, including by restricting their ability to board commercial planes, according to an administration official.

Mr. Biden has resisted calls to move the inauguration ceremony indoors for the sake of safety. His inauguration committee had already been planning a scaled-back celebration with virtual components because of the coronavirus.