Los Angeles schools, which were closed today after threat, will re-open tomorrow

- The nation's two biggest school systems - New York City and Los Angeles - received threats Tuesday of a large-scale jihadi attack with guns and bombs. LA reacted by shutting down the entire district, while New York dismissed the warning as an amateurish hoax and held classes.

It's extremely rare for a major U.S. city to close all its schools because of a threat and it reflected the lingering unease in Southern California following the terrorist attack that killed 14 people at a holiday luncheon two weeks ago in San Bernardino.

In LA, the threat came in the form of an email to a school board member. Authorities in New York reported receiving the same "generic" email and decided there was no danger to schoolchildren. Mayor Bill de Blasio concluded the threat contained "nothing credible."

"It was so outlandish," he said.

New York Police Commissioner William Bratton agreed. Bratton, who was police chief in Los Angeles until 2009, said that it looked like the sender of the threat had watched a lot of the Showtime terrorism drama "Homeland."

Bratton indicated that the type of threats in the email mirrored some recent episodes of the show.

The shutdown closed both public and charter schools across Los Angeles. Officials announced Tuesday evening that schools would reopen Wednesday.

Officials in LA defended the move to shut schools, with that city's police chief dismissing the criticism as "irresponsible."

"We have suffered too many school shootings in America to ignore these kinds of threats," Chief Charlie Beck said at a news conference.

Jordan Tama, an assistant professor at American University specializing in U.S. counterterrorism policy, said it's not unreasonable for authorities in different places to make different decisions based on the same information.

"There certainly is no uniform approach," he said. "Los Angeles might make their decision based on different factors than New York and that would be the case throughout the country."

Against the backdrop of the San Bernardino attack, it's "just human nature" for LA authorities to be more cautious, Tama said.

The threatening 360-word email sent to the New York City school superintendent warned that schools would be attacked with pressure cooker bombs, nerve agents and machine guns. It claimed the writer and "138 comrades" would carry out the attack.

Students "at every school in the New York City school district will be massacred, mercilessly. And there is nothing you can do to stop it," the message said.

A law enforcement official with access to the document provided the email to The Associated Press. The official was not authorized to disclose details of an ongoing investigation and provided it only on condition of anonymity.

The anonymous writer claimed to be a student at a district high school who had been bullied. The person also claimed to be a jihadist but made errors that suggested the writer was really a prankster, including spelling the word "Allah" with a lowercase "a'' and making no reference to the Quran.

The threats came in simultaneously to New York and LA school officials at about 1:20 a.m. EST Tuesday, or about 10:20 p.m. Monday in Los Angeles.

In LA, the school board member who received the threat immediately contacted school district police, Det. Rudy Perez said.

Across the country, the New York schools superintendent who received the threat was asleep and did not notice the email until 5:08 a.m. By 6:30 a.m., the message was sent to the NYPD.

An hour later, New York students began arriving at school with no knowledge of the threat. By about 9:30 a.m., investigators ruled the threat a hoax.

The decision to close Los Angeles schools was announced around the same time, at 6:25 a.m. PST.

Bratton called the closure in Los Angeles a "significant overreaction."

"We cannot allow ourselves to raise levels of fear," he said.

White House spokesman Josh Earnest said he would not second-guess the decisions made in Los Angeles or New York.

The sudden closure disrupted the routines of many Los Angeles families.

Lupita Vela, who has a daughter in the third grade and a son who is a high school senior, called the threat "absolutely terrifying" in light of the San Bernardino attack.

"I know the kids are anxious," she said.

The LA schools commonly get threats, but Los Angeles Superintendent Ramon Cortines called this one rare and said the San Bernardino attack influenced his decision to close the entire district.

The threat "was not to one school, two schools or three schools," he said at a news conference Tuesday morning. "It was many schools, not specifically identified. ... That's the reason I took the action that I did."

In announcing that schools would reopen, LA officials said more than 1,500 buildings were searched, and police patrols outside the campuses will be increased on Wednesday. Mayor Eric Garcetti said the FBI concluded that the threat wasn't credible.

The daylong shutdown kept some 640,000 students out of classes and cost the district some $29 million in state funding, officials said.

The person who sent the threat used an "anonymizer," which uses a proxy server to mask the origin of Internet traffic, and the email was routed through a German IP address, according to a law enforcement official briefed on the investigation. The official, who was not authorized to discuss an ongoing investigation, spoke on condition of anonymity.

Vela said she worries about talking to her kids about the threat and terrorism in general. She's concerned about her daughter feeling secure in class.

"I don't want this to be in the back of her head," she said. "Who knows what it does psychologically to kids? Is this going to cause her some kind of trauma so that she's not going to feel safe at school?"