LOS ANGELES -- The nation's second-largest school district shut down Tuesday after a school board member received an emailed threat that raised fears of another attack like the deadly shooting in nearby San Bernardino, California.
Authorities in New York City said they received the same threat but quickly concluded that it was a hoax.
New York Mayor Bill de Blasio told reporters that he was "absolutely convinced" there was no danger to schoolchildren in his city.
"There was nothing credible about the threat. It was so outlandish," de Blasio said.
New York Police Commissioner William Bratton agreed, quipping that it looked like the sender of the threat watched a lot of the Showtime terrorism drama "Homeland."
Bratton called the closure a "significant overreaction."
"We cannot allow ourselves to raise levels of fear," said Bratton, who once ran the Los Angeles Police Department.
Hours later, the ranking Democrat on the House Intelligence Committee also said the threat was believed to be a hoax.
U.S. Rep. Adam Schiff, D-Calif., said a preliminary investigation indicates that the threats in Los Angeles and New York City were designed to disrupt school districts in large cities.
Los Angeles officials defended the move.
"It is very easy in hindsight to criticize a decision based on results the decider could never have known," LA Police Chief Charlie Beck said at a news conference.
A law enforcement source told CBS News senior investigative producer Pat Milton that the threat was emailed to several L.A. school district board members. The shutdown abruptly closed more than 900 public schools and 187 charter schools attended by 640,000 students across Los Angeles.
CBS Los Angeles reported that robocalls to parents that went out after 7 a.m. said: "As a result of a threat received the superintendent has directed all schools to be closed today." Some parents also got separate calls from the schools themselves.
Parents and guardians were urged to bring proper ID when picking up their child or children at school, as ID "will be required."
Superintendent Ramon Cortines said every campus would be searched, and he asked for a report on the searches certifying that all buildings are safe.
The threat, the New York officials said, came in the form of a "generic" email to many cities around the country. In New York, it was received by a superintendent early Tuesday.
The person who wrote the note, Bratton said, claimed to be a jihadist but made errors that indicated the writer was really a prankster, including spelling the word "Allah" with a lowercase "a."
White House spokesman Josh Earnest said he would not second-guess the decisions made in Los Angeles or New York. He said the FBI has been in touch with California authorities.
The decision to close the district disrupted the morning 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, which killed 14 people earlier this month.
She got an automated phone call informing her of the closure.
"I know the kids are anxious," she said.
District spokeswoman Shannon Haber said the threat was sent by email to a school board member and was believed to have come from an IP address in Frankfurt, Germany.
Beck said the email was specific to all the campuses in the district and included implied threats about explosive devices, assault rifles and machine pistols.
The city schools commonly get threats, but Cortines called this one rare.
"It was not to one school, two schools or three schools," he said at a news conference. "It was many schools, not specifically identified. But there were many schools. That's the reason I took the action that I did."
Milton reports that according to a law enforcement source, the email was lengthy and sometimes rambling.
"Every school in Los Angeles school district is being targeted. We have bombs hidden in backpacks in lockers at several schools and they are strategically placed to crumble the foundations of the very buildings that monger so much hate...," was one excerpt from the email read.
The San Bernardino attack influenced the decision to close the entire district, Cortines said.
The superintendent said the district police chief informed him about the threat shortly after 5 a.m.
"He shared with me that some of the details talked about backpacks, talked about other packages," Cortines said.
Vela said she worries about talking to her kids about the threat and terrorism in general. She's concerned about her daughter feeling safe 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?"
The closure came the same day classes were canceled at San Bernardino Valley College because of a bomb threat.
CBS Los Angeles spoke to one parent at Hollywood High School who said she couldn't locate her kids following news of the threat.
"I'm so scared, that's why we drive here, but they're not here," she said.
Other parents took to social media to react to the threat:
CBS News' homeland security correspondent Jeff Pegues reports that experts say ISIS in particular sends anywhere from 60,000 to 100,000 tweets a day.
Michael Morrell, former deputy director of the CIA and a CBS News consultant, said in the age of ISIS getting specific or credible threat information is a little harder, because there are so many voices out there.
"Social media...ISIS itself, and then there's ISIS's supporters, and...people saying different things, and you got people making different threats, so I think it's probably a little harder today to sort fact from fiction," he said.