All schools in Los Angeles, the second largest school district in the United States, were closed on Tuesday after officials reported receiving an unspecified threat to the district and ordered a search of all schools in the city.
Officials asked parents to keep all of the system's 643,000 students at home to allow time for a full search of more than 1,200 schools, from primary through high schools. It was the first closure of the full district in at least a decade, officials said.
The threat came less than two weeks after a married couple inspired by Islamic State militants shot dead 14 people in San Bernardino, California, about 60 miles (100 km) east of Los Angeles.
"It was not to one school, two schools or three schools, it was many schools, not specifically identified ... that's the reason I took the action that I did," Ramon Cortines, superintendent of the school system, told reporters at a press conference that began shortly before schools were to begin opening.
"I am not taking the chance of taking children any place into the building until I know it's safe."
Los Angeles police and the FBI were notified of the threat and were investigating, officials said.
The threat came via an electronic message, Cortines said.
Officials said they were not aware of any other threats to schools outside the district, adding that they would issue additional details on the threat later in the day.