A teenager admitted rigging pens that exploded and injured three people at two high schools because he wanted revenge after being expelled, a detective said.

El Monte police arrested the 17-year-old boy on Wednesday after an anonymous phone tip led them to search his home.

They said they found explosive materials and books he allegedly used to learn how to booby-trap the pens to detonate when uncapped. Victims who found them lying on the ground at Rosemead and El Monte high schools suffered minor burns and scratches on their hands and faces.

"He admitted this was stupid," said El Monte Police Det. Dennis Miller. "It's a very serious offense. I wouldn't consider it a prank. ... It could have turned into something that caused far more injury, and even death, than it did."

The name of the teen, who had attended Rosemead, was not released because he is a minor. Miller and school officials declined to say when or why he was expelled.

He was in custody on suspicion of using a destructive device and possession of bomb-making components, both felonies. The district attorney was to get the case later Friday to decide whether to bring charges.

In the first incident, at Rosemead High School on Aug. 24, a construction worker picked up a pen near a line of students. In September, a Rosemead resident picked up a pen outside the school's fence. Last week, a student picked up a felt-tip pen in a restroom at El Monte High School.