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Officials closed all Los Angeles Unified School District campuses Tuesday morning after receiving a "credible threat" of violence involving backpacks and packages left at campuses.

Authorities said they plan a search operation of all of LAUSD's more than 900 schools, including charter schools and special education centers. The nation's second-largest school district, LAUSD has more than 700,000 students. 

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“I think it’s important to take this precaution based on what has happened recently and what has happened in the past,” LAUSD Superintendent Ramon Cortines said.

The move comes less than two weeks after two shooters killed 14 people in San Bernardino in what was the deadliest terrorist attack on U.S. soil since Sept. 11, 2001. 

Fearing the safety of schools and students, Cortines said he couldn’t take a chance, so he asked police to search all campuses, adult school and early-education centers before reopening Wednesday.

Officials said the threat came in electronic form and was made to numerous but unspecified campuses. As a result, they made the decision to close all campuses for the day.

The Los Angeles Police Department and FBI were assisting with the threat investigation, Los Angeles School Police Chief Steve Zipperman said.

'Credible threat' to Los Angeles school district prompts closures

“The threat is still being analyzed,” he said. “We have chosen to close our schools today until we can be sure our campuses are safe.”

Students who already arrived at school will be supervised until parents can pick them up, officials said. LAUSD ordered parents and guardians to bring identification with them.

Brian Levin, a terror expert at Cal State San Bernardino, said such a closure was unprecedented and could embolden others to make future threats.

"In today's environment it makes sense to err on the side of safety, even though they almost always are hoaxes," he said.

New York authorities were also responding to a threat made to city schools, but said it was not credible and that they were concerned about overreacting.

"These threats are made to promote fear...we can not allow us to raise the levels of fear," New York City Police Commissioner William Bratton tweeted.

He added that their agency was working closely with the FBI and the LAPD.

Zayda Hernandez pulled up to Mayberry Elementary in Echo Park shortly after 7:30 a.m. with her 6-year-old son, Matthew Alvarez, bundled up in the back seat in a coat and SpongeBob stocking cap.

He hadn't been feeling well, so she has been urging him to just make it through the last few days of school before winter break.

She pulled up to see paper signs with purple writing attached to the closed chain-link fence outside the school: “No school today.” “Hoy no hay escuela.”

“No school!” she said, rolling down her window, shaking her head.

She was driving from her home near Chinatown on Tuesday morning when she heard on Spanish radio 107.5 that schools would be closed. But it was so late and there was a private school she knew of that was open, so she wanted to check just to be sure.

“I pulled up and thought, 'There's no traffic so maybe it's true.' ”

She showed an alert she got on her smartphone at 7:22 a.m. from Mayberry administrators saying that at 7:10 a.m., the superintendent decided to close schools. “Do not send your child to school. Please watch the news for further updates,” it read.

Hernandez was not happy about the closure because she has to go to work. But her son, a kindergartener, grinned.

Cortines said a statement will be issued later Tuesday, providing an update on the investigation.

The “rare” threat message was made to students at many schools, he said.

“What we are doing today is no different than what we normally do, except that we are doing it in a mass way,” Cortines said.

LAPD Assistant Chief Jorge Villegas said police are taking the threat, like all threats, seriously.

“Nothing is [more] important to us than the safety of our kids, especially those that are coming to and from school that haven’t been notified yet,” he said.

LAUSD Board President Steve Zimmer urged families not to send their children to schools and employees to stay home.

“We are taking this action in an abundance of caution to make sure that every child in L.A. Unified School District and every employee is absolutely safe,” Villegas said

If students already were dropped off Tuesday morning at LAUSD campuses, parents must pick up their children at the schools’ reunion gates.

“I want to be very clear: We need cooperation of the whole of Los Angeles today,” Zimmer said, pleading for employers to show patience for parents looking to find care for their children.

Hope Street Friends, a private daycare and preschool in the Wells Fargo complex in downtown L.A. decided to follow LAUSD's lead and close Tuesday.

As a result of the closures, Metro announced that students with a valid student ID can ride buses and trains for free until noon.

Times staff writer Harriet Ryan contributed to this report.

For breaking news in California, follow @VeronicaRochaLA

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