After questioning four students, police arrested 18-year-old Mark Woods, a freshman economics major from Torrance. University spokeswoman Lisa Lapin said he has been cooperating with authorities. No other arrests were expected.
Woods was being held on possession of materials with the intent to make an explosive or destructive device and other charges and possession with intent to make such a device on school grounds. It was not clear if he had a lawyer.
"Police investigators say there is nothing to indicate that this case was related to a terrorist act," a university statement said. "The investigation is continuing."
Police became aware of the situation Wednesday night after a woman reported that students had possible explosive devices.
Two partially assembled pipe bombs were found in Woods' dorm room, Lapin said. She said a roommate said he was curious and liked to experiment but was a serious student and would have had no criminal intent.
More than 450 students had to spend the night in dining halls or at the homes of family and friends. But with no imminent danger of explosions, police waited until Thursday morning to get a search warrant, which requires a judge's signature. Classes were not canceled.
The students were expected to be let back into their dorm rooms later Thursday morning, Lapin said.