After shooting his grandparents, the teenager boarded the school van that came to fetch him at home, said Thadsakorn. He carried the gun and dozens of bullets with him.
The Debsirin Nonthaburi School, where the boy studied, lies around 18km from the house, or a 45-minute drive. Last year, the school had an enrolment of around 3,100 students, taught by 147 teachers, according to district authorities.
His first class began at 8.30am, followed by another at 9.20am.
About half an hour into the boy’s second class of the day, he drew out the gun and began shooting again, police estimate.
At 9.57am, the Plai Bang Police Station overseeing the area where the school is located received a phone call about a shooting, an official said.
As the shots rang out, one 18-year-old student initially thought he was hearing fireworks going off, before a teacher told him to evacuate. Some ran out in their socks or bare feet, clutching sheets of paper and mobile phones.
Others took cover under their desks, weeping as they lay on the floor listening to the sound of the firing, according to a video posted on social media and verified by Reuters.
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