LOS ANGELES (AP) — All schools in the vast Los Angeles Unified School District, the nation's second largest, have been ordered closed due to an electronic threat Tuesday.

During a press conference Tuesday, district police Chief Steven Zipperman said the threat was still being evaluated. Schools would remain closed until the threat was cleared, which officials said could happen by the end of the day.

"We need the cooperation of the whole of Los Angeles today," said school board President Steve Zimmer. "We need families and neighbors to work together with our schools and with our employees to make sure our schools are safe throughout today."

District Superintendent Ramon Cortines said the Dec. 2 attack in nearby San Bernardino that left 14 people dead influenced the decision.

He would not detail the threat but described it generally as a "message."

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

He says the schools commonly get threats, but called this one rare.

Cortines said the district police chief informed him about the threat shortly after 5 a.m.

"He shared with me that some of the details talked about backpacks, talked about other packages," said Cortines, who would not elaborate.

The district has 640,000 students in kindergarten through 12th grade and more than 900 schools and 187 public charter schools.

The district spans 720 square miles including Los Angeles and all or part of more than 30 smaller cities and some unincorporated areas.

The closure came the same day classes were canceled at San Bernardino Valley College because of a bomb threat. Students and staff were sent home around 5:30 p.m. Monday after the threat was made.