Over 600,000 public students across Los Angeles have been sent home as authorities carry out room-by-room searches at around a thousand schools.
The threat was delivered by email to school board members.
Los Angeles Police Chief Charlie Beck says all public schools in the district were threatened.
"The implied threat was explosive devises. The specific threat was attack with assault rifles and machine pistols. We worked with the FBI to vet this as best as possible. The original document was routed through a European country, Germany. But the origin has yet to be determined and we believe to be much closer."
The superintendent of Los Angeles schools, Ramon Cortines, says the unprecedented decision to shut more than a thousand public schools was influenced by the deadly shooting in nearby San Bernardino less than two weeks ago.
"I can't take a chance. And all schools I have asked to be searched today by the appropriate people and the appropriate people are not necessarily teachers or support people. If there is a problem I want our Chief of Police working with our City Police, and working with the Sheriff to develop a plan. We have 6 districts to work through systematically through every one of those schools."
Students already at school were sent home, and offered free travel on buses and trains.
This student was sent homer by his principal.
"It's kind of bad because this is finals week and we're supposed to be studying for our finals so, we're supposed to be showing up as you can see, but it's kind of messing our studies up."
As hundreds of thousands of families in LA scrambled to make alternative care arrangements, there was news that New York had received a similar threat.
But the mayor there, Bill de Blasio, says it was deemed a hoax.
"There was nothing credible about the threat. It was so generic. So outlandish. And posed to numerous school systems simultaneously."
LA's mayor, Eric Garcetti, forced to defend his city's dramatic shutdown, says his state's decision was taken with "an abundance of caution".
"We will continue to hope that this is nothing and that our children can be back at school tomorrow. But as a parent and as a mayor I am here to support this school district as it seeks our help to ensure that we can look at each one of these campuses and make sure they are safe for all of our children."
The United States frequently has suffered many deadly school attacks, typically carried out by gunmen.
In 2007 a shooter killed 32 people at Virginia Tech.
And three years ago 26 people were killed at Sandy Hook Elementary School in Connecticut.
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