The Los Angeles Unified School District cancelled all classes and closed all schools on Tuesday after receiving an unspecified threat.

Congressman Adam Schiff (D-California), who serves on the House Intelligence Committee, said that the threat now appears to be a hoax.

“The investigation into LAUSD threat is still ongoing,” Schiff wrote on his verified Twitter profile. “Preliminary assessment is it was a hoax to disrupt school districts in large cities.”

Speaking at a news conference on Tuesday morning, the district’s superintendent, Ramon Cortines, told reporters that the threat was made against “not one school, but many schools in this school district.”

The district’s Twitter account said the schools received a “credible threat,” but did not elaborate.

Cortines told reporters the threat involved “backpacks and other packages,” and he decided to close every school in the district, the second-largest in the nation, as a precautionary measure.

“I think it is important that I take the precaution based on what has happened recently and what has happened in the past,” Cortines said at the news conference.

The announcement came about two weeks after the Dec. 2 mass shooting in nearby San Bernardino in which 14 people were killed. The shooting has been called an act of terrorism.

According to its website, the Los Angeles Unified School District has more than 900 schools and more than 640,000 students.

Cortines said he wanted every school in the district to be searched by the end of the day.

New York Police Commissioner Bill Bratton told reporters in New York that the city’s schools received a similar threat Tuesday, but determined it was not credible.