An unspecified threat was reported in Los Angeles, which resulted in closing of all the schools in the district on Tuesday.
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, Reuters reported.
The threat came days after a heavily armed US-born county health worker and his Pakistan-born wife killed 14 people and injured 17 others during a holiday party in California.