An "electronic threat" was made against "not one, but many schools," said a city official
Officials in Los Angeles have defended a decision to close the nation's second largest school district after receiving a "rare threat", apparently from an overseas email account.
The threat involving backpacks and "other packages" was made against "not one, but many schools" in the district, LA Schools Superintendent Ramon Cortines said.
LA Police Chief Charlie Beck said the threat also described an attack with assault rifles.
Meanwhile, officials in New York City said they received the same email threat, but concluded it was a hoax.
Mayor Bill de Blasio said he was "absolutely convinced" there was no danger to schoolchildren in New York, and the city's police commissioner said he thought LA officials overreacted.
"We are very comfortable that this is not a credible threat ... concerned with people overreacting to it," NYPD Commissioner Bill Bratton said.
He added that his department was working with the LAPD and FBI on the threat.
Los Angeles Mayor Eric Garcetti told a news conference it is "very easy for people to jump to conclusions ... but decisions need to be made in a matter of minutes".
Chief Beck added: "I think it's irresponsible, based on facts that have yet to be determined, to criticise that decision at this point."
Mr Cortines, who made the call to shut the more than 1,000 schools across the city, said: "We are doing everything possible to make sure all children are safe".
Earlier on Tuesday, the superintendent told reporters: "I can't take a chance. I've asked for all schools to be searched today."
One mother of an LA student told Sky News she believed the city-wide closure was the "responsible" thing to do.
Ila Laguericia said: "I'm very grateful that LA Unified made this decision. It's one of the largest school districts in the country and I think it's very responsible."
She added: "I appreciate they're taking this seriously - hoax or not - because we're talking about children, and in this unfortunate world we live in right now we need to keep them safe."
The threat comes amid heightened security concerns following the mass shooting in San Bernardino, about 60 miles (96km) east of downtown Los Angeles.
Speaking about his decision to close all of the city's schools, Mr Cortines referenced the attack that killed 14 people and wounded 21 others.
He said: "I'm not going to endanger the lives of students."
A law enforcement official speaking to the AP news agency on condition of anonymity said the threat was emailed to a school board member and appeared to come from overseas.
Chief Beck said the email was sent from an IP address in Frankfurt, Germany, but investigators believe its origin was much closer. He did not elaborate.
Mr Cortines said the district regularly deals with threats, but he described Tuesday's threat as "rare".
The Los Angeles Unified School District (LAUSD) is the nation's second largest with 640,000 students in kindergarten through 12th grade and more than 900 schools and 187 public charter schools.