Students and teachers in Los Angeles prepared to return to class Wednesday after an emailed threat of a large-scale jihadi attack with guns and bombs prompted the entire district to shut down.

Meanwhile, Sunny Hills High School in Fullerton, southeast of Los Angeles, closed its doors for the day after officials there said somebody taped a suspicious note to an administrator's door. It did not describe a bomb or terror threat, police told The Orange County Register.

Los Angeles' reaction Tuesday marked a sharp contrast to that of New York, which received a similar message threatening its schools but dismissed it as a hoax and held classes as scheduled. New York City Police Chief Bill Bratton called the response from Los Angeles a "significant overreaction." He said it looked like the sender of the threat had watched a lot of the Showtime terrorism drama "Homeland."

"We have suffered too many school shootings in America to ignore these kinds of threats," LAPD Chief Charlie Beck said at a news conference, while dismissing criticism as "irresponsible." Beck succeeded Bratton as LAPD chief in 2009.

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

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

A senior law enforcement source told Fox News late Tuesday that the threats to Los Angeles and New York were sent via an anonymous email hosting service. The email was routed through a German IP address.

The source told Fox News federal authorities deemed the threat not to be credible because the email described an unrealistic amount of resources and manpower that would be used to attack the schools, spelled "Allah" with a lower-case "a", and made no reference to the Koran.

"They claim to be [a] devout Muslim, one might say an extremist Muslim, that is now working with jihadis and he claims to have 32 accomplices and so all 33 of them are supposed to descend on Los Angeles schools [Tuesday] morning," Rep. Brad Sherman, D-Calif., told Fox News Channel's "On The Record" Tuesday evening.

A government official told Fox News the Los Angeles email also described a bomb in a backpack and other packages in the schools.

The Associated Press reported that 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 said the writer, who claimed to be a bullied high school student, 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.

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 Los Angeles, the school board member who received the threat immediately contacted school district police, LAPD Det. Rudy Perez told the Associated Press.

“I made the decision to close the schools,” LA Unified School District Superintendent Ramon Cortines confirmed Tuesday, citing the need to ensure students' safety.

The announcement came less than two weeks after a terror attack in San Bernardino, some 50 miles to the east, left 14 dead and 22 wounded.

Fox News' Catherine Herridge, Adam Housley, Michael Setsuda, Matt Dean and The Associated Press contributed to this report.