Public schools in Los Angeles are set to reopen a day after local officials cancelled class for about 640,000 students in the nation's second-largest school district over a threatened attack with bombs and guns later deemed a hoax.

Authorities conducted an extensive search of the Los Angeles Unified School District's more than 1000 schools and by late Tuesday said the buildings were secure and students were safe to return.

The emailed threat, which authorities said was routed through Germany but likely originated locally, came nearly two weeks after a married couple inspired by Islamic State killed 14 people and wounded 22 others at a county office building 100km away in San Bernardino.

A similar email was sent to New York City's public schools though officials dismissed it as a hoax and kept campuses open.

Los Angeles Mayor Eric Garcetti said an investigation was in its early stages.

"Whether it's criminal mischief, whether it's somebody testing vulnerabilities of multiple cities, we still do not know enough to say definitively," he told a press conference on Tuesday.

Los Angeles police chief Charlie Beck defended the school district's decision to keep students and staff home out of an abundance of caution.

"It is very easy in hindsight to criticise the decision based on results that the decider could never have known," he said.

New York City police commissioner William Bratton and federal officials who asked not to be identified described the decision in Los Angeles as an "overreaction".

Beck said extra patrol officers would be deployed at city schools on Wednesday in part to manage jitters following the unprecedented closure.

The threat, which was emailed late on Monday night, came from someone who claimed to be a devout Muslim prepared to launch an attack at multiple schools using bombs, nerve gas and rifles, congressman Brad Sherman, a Democrat from California, told the New York Times.

The decision to cancel school, which cost the district an estimated $US29 million according to the Los Angeles Times, left parents scrambling to make alternate arrangements for their children.