Los Angeles schools have been closed due to a threat emailed to the school district, two weeks after the deadly attacks in California.
The shutdown closed more than 1,000 schools attended by 640,000 students across the city and beyond.
A school district spokesman said it involved rucksacks, and they were exercising "an abundance of caution".
New York City officials said they received a similar threat but it was so "outlandish" they ignored it as a hoax.
Congressman Adam Schiff, the ranking Democrat on the House Intelligence committee, said the Los Angeles email was also deemed a hoax.
New York City Police Commissioner William Bratton said the emailed threat came from overseas, purporting to be from jihadists, but was not considered credible.
He said he thought Los Angeles officials overreacted.
But LA Police Chief Charlie Beck responded by saying it was "irresponsible" to criticise when you didn't have the responsibility of safeguarding three-quarters of a million lives.
The threat came less than two weeks after 14 people were killed by a radicalised Muslim couple in San Bernardino, 60 miles east of Los Angeles.
"Earlier this morning we did receive an electronic threat that mentions the safety of our schools," said Steven Zipperman, chief of the Los Angeles school police department.
"We have chosen to close our schools today until we can be absolutely sure that our campuses are safe."
An unnamed police official told Associated Press news agency the email sent to New York City threatened an attack on "every school in the city" with pressure cooker bombs, nerve gas agents, machine pistols and machine guns.
In Los Angeles, the threatening email described an attack specific to the Los Angeles Unified School District with assault rifles and explosive devices, said Mr Beck.
In a reference to the San Bernardino tragedy, district superintendent Ramon Cortines said: "I think it's important to take this precaution based on what has happened recently and what has happened in the past."
Los Angeles Unified School District
second largest in the US
640,000 students, about 1,087 schools
district spans 720 sq miles (1,865 sq km)
includes Los Angeles and all or part of more than 30 smaller cities
Officials notified parents to keep children at home or pick them up.
Lupita Vela, parent of an eight-year-old daughter who attends school in the district, told the AP news agency she was "terrified" getting the announcement, especially following the San Bernardino attacks.
She wants her daughter to feel safe at school, she said.
Another parent of a LAUSD student, Elinor McMillan, said she's a "nervous wreck".
In a separate incident, classes were also cancelled at San Bernardino Valley College on Tuesday due to a bomb threat made on Monday afternoon.
What's a credible threat?
FBI officials use the term "specific but non credible" when they have details about the location of the threat, such as a school or a house. But the officials can't say for sure that the person who made the threat is dangerous (and it's not a schoolboy prank).
"Specific means a very clear target," Frank Cilluffo, the director of George Washington University's homeland security policy institute, told me.
There are other cases when militants or criminals, known to them, make a threat, but they haven't said where the attack would take place.
That's when law-enforcement officials talk about a threat that is "credible but not specific".