LOS ANGELES — The Los Angeles Unified School District has closed all 900 schools on Tuesday due to what has been described as a “credible threat of violence” made against several Los Angeles schools.

A threat involving backpacks and packages left at campuses was emailed from Frankfurt, Germany, to a school board official on Monday evening, CBSLA reports. The threat was made to several unspecified schools in the district, though no other area districts reported receiving any threats. In response, district officials decided to close all campuses for the day, according to the Los Angeles Times.

Authorities said in a news conference held in Los Angeles on Tuesday that they are searching all of the district’s schools, including charter schools and special education centers. Meanwhile, parents who had already dropped off their children at their school were asked to pick them up, or to keep them at home if they had not already gone to school. All school buses have been recalled to bus yards, KPCC reports. The Los Angeles public transit system is offering free rides to students whose parents are not able to pick them up from school.

District Superintendent Ramon Cortines said at a news conference that the threat was made to students, and that he thinks taking the precaution is important because of “what has happened recently and what has happened in the past” the Los Angeles Times reports.

The threat comes on the heels of the mass shooting in San Bernardino on Dec. 2, in which two shooters killed 14 people.

The second-largest school district in the nation, Los Angeles Unified has over 640,000 students, according to the district’s website.

Los Angeles Police Department Chief Charlie Beck said in a news conference that the threat involved using explosives and firearms, and that the Los Angeles Police Department and the FBI are working together in the threat investigation.

However, New York City officials received the same threat, but concluded that it was a hoax, and did not send students home, according to CNN. Former LAPD chief and current New York Police Commissioner William Bratton said he thought the school closures was an overreaction. He added that a note that was sent by someone who claimed to be a jihadist made errors that showed the person was a prankster, KPCC reports. Officials said in the news conference that the threat was made to other places in addition to Los Angeles and New York.

Beck also said in the news conference in response to critics of the school closures that it is easy to criticize what he described as a "high-stakes decision" in hindsight when you are not responsible for the outcome of that decision. Additionally, White House Press Secretary Josh Earnest said that the Los Angeles school district had the support of the FBI to close all schools and that the Bureau of Alcohol, Tobacco, Firearms and Explosives is investigating the threat.