Source: BBCNews
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 the decision was made out of "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.
Los Angeles Mayor Eric Garcetti said the FBI had confirmed it was not a credible threat. Schools will reopen Wednesday.
New York City Police Commissioner William Bratton accused his Los Angeles counterparts of overreacting.
But LA Police Chief Charlie Beck dismissed the criticism as "irresponsible".
"It is very easy in hindsight to criticise a decision based on results the decider could never have known,'' he said.
The threat came less than two weeks after 14 people were killed by a radicalised Muslim couple in San Bernardino, 60 miles (95km) 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