LOS ANGELES -- There are more than 1,000 schools in Los Angeles -- second only to New York -- in a district that covers 710,000 square miles.
The robocall alert from the nation's second-largest school district went out at 6:32 a.m. -- idling hundreds of school buses and stopping parents and more than 640,000 students in their tracks.
Nicole Scates' son Donovan is in second grade.
"Kind of scared, but you know, my immediate thought was just how I was going to talk to him about it," Scates said.
Eleven-year-old Sullivanne Montagu heard the news from her dad.
"He told me that it was a threat -- possibly a bomb -- and I got kind of nervous," she said.
Los Angeles Unified School District Superintendent Ramon Cortines said he was comfortable with his decision.
"Somebody has sent information that leads us to pause and make sure that we are safe -- that our children and our staff are safe," he said.
Hundreds of law enforcement officers were deployed to campuses. Police secured buildings, armored vehicles were mobilized, and every school in the district was searched.
Given the terrorist attack less than two weeks ago in nearby San Bernardino, Los Angeles Mayor Eric Garcetti defended the decision to quickly close Los Angeles schools.
"We've just gone through a horrible experience of losing loved ones, of seeing that sort of horror right here in our backyard," said Garcetti. "And so I think just as New York did after 9/11, Boston after the marathon, there is an abundance of caution that may be regionally specific."
Ninety-five percent of the Los Angeles public schools have now been swept.