A threat from a student forced the evacuation Thursday of a community college south of San Diego with about 20,000 students and 1,000 staff members on its main campus and three nearby satellite campuses.

Campus police were investigating the threat at about 8 a.m. and did not disclose information about the nature of the threat or how it was made, said Ernest Rivera, a spokesman for Southwestern College, which has a main campus in the city of Chula Vista and three other locations in neighboring communities.

Southwestern College Superintendent Kindred Murillo said the threat was from a student and considered "credible."

All classes were canceled for the rest of the day and all campuses and departments closed in "an abundance of caution while we investigate," Murillo said in a statement.

Yvonne Lucas, a business professor at the main campus, said she was working in her office when she was told about the evacuation. Thousands of people left the campus.

"People were calm, but I was concerned about the mass of people waiting for the buses and trying to get out of the parking lots," she said. "It wasn't mayhem in the sense of people panicking but you just couldn't get off campus."