Los Angeles schools have been closed as an unspecified threat is investigated, police have said.
Officials would not specify the nature of the threat, which led to students being sent home and school buses returned to depots.
A school district spokesman said it involved rucksacks, and they were exercising “an abundance of caution”.
The district is the second largest in the US, with 640,000 students attending more than 1,000 schools.
Police are searching all the schools, which will remain closed until the threat is over, officers told reporters at a press conference.
“Earlier this morning we did receive an electronic threat that mentions the safety of our schools,” said Steven Zipperman, chief of the Los Angeles school police department.
“We have chosen to close our schools today until we can be absolutely sure that our campuses are safe.”
An unnamed police official told Associated Press news agency the threat was emailed to a school board member and was thought to have come from overseas.
It comes amid a heightened alert after 14 people were killed by a radicalised Muslim couple in San Bernardino, 60 miles east of Los Angeles.
“I think it’s important to take this precaution based on what has happened recently and what has happened in the past,” said district superintendent Ramon Cortines.
“It was not to one school, two schools or three schools. It was to many schools, not specifically identified. But there were many schools. That’s the reason I took the action I did… It was to students at schools.”
When asked what was meant be “electronic” threat, he said simply that “it was a message”.
In a separate incident, classes were also cancelled at San Bernardino Valley College on Tuesday due to a bomb threat made on Monday afternoon.