A sign at Hamilton High School is pictured reading "School Closed" in Los Angeles, California December 15, 2015. All schools in Los Angeles, the second largest school district in the United States, were closed on Tuesday after officials reported receiving an unspecified threat to the district and ordered a search of all schools in the city. [Photo: Xinhua/Jonathan Alcorn]

All public schools in neighboring Los Angeles were shut down on Tuesday over bomb threats.

The move was made after authorities of the L.A. Unified School District, the second largest in the country, received threats against several schools.

650 thousand students from kindergarten through high school were told to stay away from all schools in Los Angeles.

Superintendent of the L.A. Unified School District, Ramon Cortines, says threats were made against several schools.

"It was not to one school, two schools or three schools. It was many schools. Not specifically identified. But there are many schools. That's reason I took the action that I did."

Law enforcement has been called in to search all 900 public schools and 187 charter schools to make sure that students and staffers will be able to return to safe campuses.

The threat comes amid a heightened alert after the terrorist attack in San Bernardino, California that killed 14 people and wounded 22 others two weeks ago.

However, the decision to close over one thousand schools raised the ire of some critics, as similar threats were also sent to New York but were dismissed as not credible.

New York Police Commissioner William Bratton.

"We do have an investigation underway, but it's an investigation into a hoax which we strongly believe that the email that was received in New York City which is similar and almost the same as received in other locals, specifically in Los Angeles. We do not see that as a credible terrorist threat."

New York has the country's largest public school system, with nearly one million students.

But law enforcement and officials in Los Angeles defended their decision and said the safety of citizens is top priority.