All public schools in the Los Angeles area have been ordered closed after an emailed threat targeted students.
The shooting in nearby San Bernardino that left 14 people dead this month influenced the decision to close all the schools, which 640,000 students attend, Superintendent Ramon Cortines said.
A law enforcement official said the threat was emailed to a school board member late on Monday and appeared to come from overseas.
Officials would not elaborate on the threat, saying it was still being evaluated, but said the shutdown came as a precaution.
Schools would remain closed until the threat was cleared, which could happen by the end of Tuesday.
Los Angeles schools commonly get threats, but Mr Cortines called this one rare.
He said: “It was not to one school, two schools or three schools.
“It was many schools, not specifically identified. But there were many schools. That’s the reason I took the action that I did ... It was to students at schools.”
Mr Cortines said he wants every campus to be searched and a report given to him and the school board that they are safe.
The district has more than 900 schools and 187 public charter schools.
Mr Cortines said the district police chief informed him about the threat shortly after 5am.
He added: “He shared with me that some of the details talked about backpacks, talked about other packages.”
No students would be released on their own, and school leaders would wait with children whose parents had not yet arrived to pick them up, he said.
The closure came the same day classes were cancelled at San Bernardino Valley College because of a bomb threat.
Students and staff were sent home around 5.30pm on Monday after the threat was made.
Meanwhile, New York City officials said they received the same threat that led to the closure of the Los Angeles schools but quickly concluded that it was a hoax.
Mayor Bill de Blasio said that he was “absolutely convinced” there was no danger to schoolchildren in New York.
New York Police commissioner William Bratton said he thought the Los Angeles officials overreacted by deciding to close the nation’s second-largest school system.
He said a school superintendent received the threatening email on Tuesday morning. The person who wrote the note claimed to be a jihadist but made errors that made it clear the person was a prankster, he added.
Los Angeles police chief Charlie Beck said the emailed threat described an attack with assault rifles.
He said it was specific to all the campuses in the Los Angeles Unified School District and the email’s “implied threat” involved explosive devices and the “specific threat” was a shooting attack.
Beck said the email was routed through Germany but that police believe its origin was much closer.
The police chief said the LAPD gave advice to district officials, who then chose to close all of its schools. Beck defended that decision.