"L.A. is a huge school system," said Bratton, who had previously served as police chief in Los Angeles. "To disrupt the daily schedules of half a million school children, their parents, day care, buses based on an anonymous email, without consultation, if in fact, consultation did not occur with law enforcement authorities, I think it was a significant over reaction."

Garcetti denied that assertion, saying his city had contacted federal law enforcement officials.

Congressman Brad Sherman, a Democrat from California, told the New York Times that the person who sent the email claimed to be a devout Muslim prepared to launch an attack using bombs, nerve gas and rifles with "32 jihadist friends" because he had been bullied at a Los Angeles high school.

Sherman told the paper that the number of attackers and claim to have nerve gas cast doubts on the credibility of the email, as did the writer consistently failing to capitalize the word "Allah."

"While we continue to gather information about the threat made against the Los Angeles and New York School Departments, the preliminary assessment is that it was a hoax or something designed to disrupt school districts in large cities," Rep. Adam Schiff, the top Democrat on the U.S. House of Representatives intelligence committee, said in a written statement.

Cortines, in defending his decision to take such a dramatic step, said the threat stood out from most that the district received in its seriousness and scope, referencing multiple campuses and mentioning backpacks and other packages.

"It is very easy for people to jump to conclusions and I have been around long enough to know that usually what people think in the first few hours is not what plays out in later hours," said the mayor, Garcetti. "But decisions have to be made in a matter of minutes."

Police Chief Beck said it was "irresponsible" to criticize the decision in the aftermath of the Dec. 2 attack on a regional center in San Bernardino, California, east of Los Angeles.

That massacre and other mass shootings have pushed the issues of militant Islamism and gun violence to the forefront of the U.S. presidential campaign.

A school district spokeswoman said that it had asked for 13 enforcement agencies to help search some 1,000 campuses, including 187 charter schools.

Professor Brian Levin, an expert on counter-terrorism and hate crimes at Cal State University San Bernardino, remarked on what a massive undertaking it would be to search schools.

"God bless 'em, but I couldn't do it. Who knows? Maybe they can. It involves looking in classrooms, closets, lockers - if you can get bomb-sniffing dogs in there, doing that - vehicles and surrounding perimeter areas," Levin said.

"If it were me, if I were chief, I'd want more time. But maybe the political pressures don't allow for that," he said.

Some parents used social media to vent frustration at having learned about the closures from the news media, rather than directly from the schools.

Ronna Bronstein, who has two sons in grade school, said she was trying to find out more while shielding her younger child from the news.

"I don't want him to be frightened to go back to school tomorrow," she said.