Authorities searched some 900 schools during the shutdown of the nation’s second largest school district, which appeared to be unprecedented in scale.
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, Supt Ramon Cortines said.
A law enforcement official said the threat was emailed to a school board member and appeared to come from overseas.
Officials said the shutdown came as a precaution and schools would remain closed until the threat was cleared.
The city’s schools commonly get threats, but 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.”
Cortines said he wanted every campus to be searched and a report given to him and the school board that they were safe.
The district has more than 900 schools and 187 public charter schools.
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 were released on their own, and school leaders waited with children whose parents could not immediately pick them up, said Cortines.
The closure came after 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 that 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 over-reacted by deciding to close the nation’s second-largest school system.
He said a school superintendent received the email threat yesterday 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.