A spokeswoman of Los Angeles police department confirmed that the Los Angeles Unified School District (LAUSD ) received a threat.

Police, though, could not confirm the validity or specificity of the threat.

The threat was made to “students at school,” LAUSD Superintendent Ramon Cortines said at a press conference Tuesday morning.

‘It was a message,” he said but refused to elaborate while the investigation is underway.

“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.”

The district is the second largest in the US with 640,000 students and more than 900 schools.