Arcadia High School officials say a non-specific note saying people would be killed on campus Tuesday caused the lockdown that started just before noon and ended several hours later.
Students were locked in their classrooms until around 2:30 p.m. when authorities approved their release.
No arrests have been made and police say investigators are working to identify who is responsible for the threat.
Richele Duffy, a parent of a student at the school, says the she was first alerted about the threat by a voicemail from the school's Principal David Vannasdall, and later a message from the district's superintendent, Joel Shawn.
"We have basically been kept in the dark, we don't know what's the threat. We don't know if it was a bomb threat, we don't if it was a gunman, we don't know if some student allegedly had a weapon. We have no clue what it was because nobody told us anything, other than they've locked our children up," Duffy said before her child was released.
Valerie McKinnon, a parent of two students at the school, said her daughter texted her several times during the lockdown: "Mom I want to go home I'm scared" and "Someone left a note on the deans desk that all students will die on 11.01.11."
Senior Brittnie Luu was in a hallway when the lockdown bell rang. She says a teacher from a nearby classroom pulled her and a friend inside and locked the door. “We had to turn off all the lights, we couldn’t text, we couldn’t go on the computers," she said. "For a good twenty minutes we weren’t allowed to do anything — just be silent to make sure that they got everything in order, but after that we were able to call our parents.”
The Pasadena Star-News reported police set up a command post during the lock down and blocked surrounding roads.
The school enrolled about 3,600 students last year.
This story has been updated.
Correction: We reported too soon that the lockdown was lifted around 2:20 p.m. It had not officially lifted at that time. Apologies.