Every school in the Los Angeles Unified School District (LAUSD) was closed Tuesday because of what officials described as a "credible" threat of violence against the district.

On Tuesday afternoon, Reuters reported that the threat was most likely a "hoax," according to several unnamed federal officials.

The bomb threat came in the form of an email sent to a school board member and tied to an IP address traced to Germany, according to the Los Angeles Times.

School officials had been looking into the threat since Monday at 10 p.m. local time. The email included an implied threat of explosive devices and a specific threat of an attack with assault rifles and machine pistols, according to the Los Angeles Police Department (LAPD).

"The school district received a safety threat that we’re in the process of evaluating and vetting … As you can imagine, we take all threats seriously," Jorge Villegas, the assistant chief of the LAPD, said at a press conference Tuesday morning.

Students were all sent home on Tuesday morning, according to NBC Los Angeles. The LAUSD is the second-largest school district in the US with more than 100,000 schools and over 700,000 kids. Each of those schools will be searched. The FBI is assisting with the investigation, which involved an electronic threat as well as backpacks and packages left on campus, according to the Los Angeles Times.

A lock holds the gate shut at Edward Roybal High School in Los Angeles, on Tuesday morning, Dec. 15, 2015. AP Photo/Richard Vogel “The actions that we are taking today are swift and they are appropriate … The education of our kids is incredibly important. The only thing that’s more important is their safety,” Steve Zimmer, the president of the LAUSD school board, said.

Later, Zimmer called the threat "an almost unprecedented situation."

"The threat's a serious threat," he said, later acknowledging that it would be a "long day" since every school would have to be searched.

When asked why LAUSD took the step of closing all schools, LAUSD Superintendent Ramon Cortines mentioned the terrorist attack in San Bernardino, California, less than two weeks ago, in which 14 people were killed.

"I, as superintendent, am not going to take chances with the life of the student," Cortines added.

A police officer puts up yellow tape to close the school outside of Edward Roybal High School in Los Angeles, on Tuesday morning, Dec. 15, 2015. AP Photo/Richard Vogel

Brian Levin, a terror expert at Cal State San Bernardino, told the Los Angeles Times that the mass school closure could prompt more threats.

"In today's environment it makes sense to err on the side of safety, even though they almost always are hoaxes," Levin told the Los Angeles Times.

Also on Tuesday, the New York Police Department (NYPD) tweeted that there had been a "specific but NON credible threat made to NYC schools this morning."

"We don't see this as a credible terrorist threat," NYPD Police Commissioner William Bratton said. "These threats are made to promote fear ... we cannot allow us to raise the levels of fear."

Bratton added that the NYPD was "very comfortable" this wasn't a credible threat and that they were concerned that people were "overreacting" to it.

Late Tuesday night, the district announced that the schools would reopen as usual on Wednesday.

This story is developing. Check back for more information.