Schools in Los Angeles, the second largest district in the US, were closed today after the city received a terror threat to many schools, education and police officials said.
The threat was aimed at "all schools" and "at school students generally", the officials said.
Schools are now being searched and the police and FBI are investigating, although officials have not confirmed the exact nature of the threat.
The threat came via an electronic message, Ramon Cortines, the superintendent of Los Angeles schools, said.
"In an abundance of caution as superintendent, Cortines has indicated we have decided to close our schools today until we can be actually certain that our schools are deemed safe,” Los Angeles school police chief Steven Zipperman said.
Parents were asked not to bring their students to school.
A statement will be issued later today, Cortines said.
The Los Angeles Unified School District has more than 640,000 students from kindergarten to 12th grade at over 900 schools and 187 public charter schools.