“I think it is important that I take the precaution based on what has happened recently, and what has happened in the past,” said LAUSD Superintendent Ramon Cortines in a press conference Tuesday morning.

He was speaking of his decision to shut down all schools in the Los Angeles Unified School District in response to a safety threat — outlets including the Los Angeles Times are reporting it as a bomb threat.

“I have alerted all six superintendents … I’ve asked them to have their operations people talk to the plant managers at each school. I’ve asked the plant managers to lock the school, and if they see anything that is out of order to contact the police, not to touch anything, not to do anything. But if they see anything out of line, to contact the proper authorities,” continued Cortines. He also spoke of concerns over students who walk to school, and ensuring that they also returned home safely.

About 640,000 students are affected by the school closures. CNBC reports Los Angeles has the second largest school district in the nation, with over 900 schools and 187 public charter schools.

LAUSD President Steve K. Zimmer asked for understanding from employers considering that many parents will be affected by the school shut down: “We ask you to show the maximum possible flexibility with your employees who are primarily mothers, fathers and guardians today in this situation.”