"Very little description of this person was provided," he said.
One student reported that another was armed, Pasadena Unified School District spokeswoman Binti Harvey said.
As a precaution, all 1,200 students were confined to classrooms for hours as police searched the campus, formerly called Blair High School. Anxious parents waited outside for hours until their children began being released after 5 p.m.
Police handcuffed and detained a youth but would not say why. Pederson, who said police weren't sure whether he was a student, called him a "person of interest."
Blair East, an adjoining middle school, also was locked down; those students were released at midafternoon. Officials did not search off the campus.
Students were individually searched, then taken in small groups to a gymnasium, walking with hands on their heads. Some parents watching from afar wondered why it was necessary or said they were frustrated by a lack of information.
"Nobody knows what's going on," said Tami Waters, mother of an eighth-grader at the middle school. "I think it's causing a lot more stress than necessary."
Pasadena is about 15 miles northeast of downtown Los Angeles.
