The boy set off the bombs around 8 a.m. after walking into Hillsdale High School in San Mateo with at least 10 explosive devices attached to a vest he was wearing, said police Lt. Mike Brunicardi. He also was carrying a chain saw.
The name of the boy, a former student at the high school, was not released because he is a minor, authorities said. Investigators have not determined a motive.
"There's no question in our minds that this individual ... may have caused grave damage, serious injuries and casualties," San Mateo Police Chief Susan Manheimer said at a news conference. The 1,200-student school was evacuated immediately after the blast and classes were canceled for the day.
The pipe bombs detonated in a hallway near a library, authorities said. Brunicardi had no specific details on the damage but said it wasn't "drastic."
After the blasts, two teachers confronted the suspect, with a third teacher tackling the suspect outside an exit door. A principal joined them and held the teen for police, who arrived within minutes, Brunicardi said.
Brunicardi described the actions of the teachers, principal and police officers as "nothing short of heroic."
"They're jumping on a kid with active explosive devices," he said. "I call that a hero."
Police initially received reports that someone was carrying a gun on campus, then subsequent calls of a blast. Authorities never located a gun.
Students and teachers told some media outlets that power was shut off in the school before the blast. Brunicardi could not confirm the reports.
Mary Kathryn Souther, a 15-year-old sophomore at the high school, told the San Mateo County Times newspaper she was in her history class when she heard a "huge bang."
"It sounded like a gun shot," she said.
Her teacher checked on the sound, then returned to the class, locked the door and told students to sit against the wall on the other side of the classroom, Souther told the newspaper.
Brunicardi said the suspect has not attended any San Mateo Union High School District school for more than a year.
Television video showed a young man in a white T-shirt in handcuffs being placed in a police vehicle.
Investigators were preparing requests for search warrants to search the teen's home, said Brunicardi. San Mateo County prosecutors would be reviewing reports from investigators on Tuesday to determine what charges to file, he said.
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