UPDATE: Officials have determined that the threat that prompted the closure of all schools in the Los Angeles Unified School District today was not credible and the schools will reopen tomorrow. More than 1,500 sites were inspected and determined clear, said Steve Zimmer, president of the L.A. Board of Education.

PREVIOUS:  Los Angeles officials say the threat received to LA schools is still being investigated and schools will not re-open until they are deemed safe.

At a mid-morning news conference covered live by local stations and cable news nets, LA Police Chief Charlie Beck said the credible threat came in “very late last night” and was sent (via email) to various members of the school board. He said local authorities immediately notified the FBI. He also defended the decision to close the schools, saying, “It is very easy in hindsight to criticize a decision based on results the decider could never have known.”

Officials said the emailed threat was routed through Germany and warned of the use of explosive devices, assault rifles and machine gun pistols.

Schools Superintendent Ramon Cortines announced early this morning that all 900-or-so LAUSD campuses were closed for the day. The district also sent a tweet at 7:24 am announcing the closure.

At an earlier news conference, Cortines described it as a “rare threat” and that he ordered all campuses searched before the end of the day today. Cortines said he ordered the closures because he was not going to take a chance with students following the recent terror attacks in Paris and San Bernardino. Board of Education President Steve Zimmer said the district was acting “out of an abundance of caution.” LAPD and the FBI are assisting in the investigation.

The New York City Police Department also received an email similar to the threat received in Los Angeles, but it was determined to be a hoax and not credible, officials said. Schools there remain open.

LAUSD is the second-largest school district in the country, serving more than 640,000 students between kindergarten and 12th grade, according to its website. There are more than 900 schools and 187 public charter schools in the district.