Police ordered the closure of all schools in the Los Angeles Unified School District (LAUSD), a spokesperson said on Tuesday.

The precise nature of the threat was still not clear, although some local news stations were reporting it was a bomb threat and had to do with school buses. Officials also said backpacks were involved.

"Earlier this morning we did receive an electronic threat that mentions the safety of our schools," said Steven Zipperman, head of the LA school police department.

It later emerged that the emailed threat was sent from an IP address in Frankfurt, in western Germany.

Not just any threat

The district's superintendant emphasized the severity of this particular threat.

"It was not to one school, two schools or three schools, it was many schools, not specifically identified...that's the reason I took the action that I did," Ramon Cortines said at a press conference.

City braces itself amid terror fears

Rep. Adam Schiff, the ranking Democrat on the House Select Committee on Intelligence, said later on Tuesday the threat was likely a hoax.

New York City officials said they had received the same threat on Tuesday, but didn't deem it credible.The officials seemed convinced that LA had overreacted by closing all schools.

The FBI was called in to help police with their investigation shortly after the closure was announced.

The incident comes two weeks after a mass shooting in San Bernardino, California, in which a Muslim husband and wife murdered 14 people at a Christmas party. Officials in LA acknowledged the incident while talking to reporters on Tuesday.

As it's the second largest school district in the US, some 640,000 students will be affected by the closure in more than 900 schools and 187 public charter schools.

blc/msh (AP, Reuters, AFP)