A no-school sign is pictured at Florence Nightingale Middle School in the Cypress Park neighborhood of Los Angeles, California December 15, 2015. [Photo/Agencies]

WASHINGTON -- The bomb threat that shut down the Los Angeles school system was believed to be a hoax, a top Democrat on the House Select Committee on Intelligence said on Tuesday.

"The preliminary assessments is that it was a hoax or something designed to disrupt school districts in large cities," said Adam Schiff, the ranking Democrat on the House select committee in a statement after a briefing.

All schools in Los Angeles Unified School District, the nation's second-largest school district, had been shut down since Tuesday morning after a school board member received an "electronic threat" that mentioned the safety of schools in Los Angeles.

More than 900 public schools and 187 charter schools were closed as a result of what local authorities called "credible threat." It was estimated that a total of 640,000 students across Los Angeles were affected by the shutdown.

Also on Tuesday, authorities in New York City said they received similar threat but soon concluded that the threat was a hoax.

"There was nothing credible about the threat. It was so outlandish," New York Mayor Bill de Blasio told reporters.

The twin bomb threats against schools in Los Angeles and New York came less than two weeks after two shooters shot down 14 people and injured 21 at a southern California social services center.

U.S. President Barack Obama in a rare Oval Office address called the southern California shooting spree a new phase of terrorism.

"As we've become better at preventing complex multi-faceted attacks like 911, terrorists turn to less complicated acts of violence like the mass shootings that are all too common in our society," Obama said on Oct. 6.

Although the shooters' motive still remained unclear to the authorities, concerns were raised across the country over terrorist attacks inspired by the extremist group the Islamic State.