Nearly 2,000 inmates at San Quentin remained on lockdown Thursday while prison staff investigated the cause of two fights earlier this week.

The lockdown pertains to the prison's "main line" or permanent general population inmates only.

"This is pretty common," said prison spokesman Sgt. Eric Messick, referring to the partial lockdown known as a modified program. "Whenever we have disciplinary issues we go on modified program. We haven't had a full lockdown at San Quentin since the '80s."

The affected inmates are confined to their quarters and prevented from using the exercise yard, going to work or attending classes. They eat in the main dining hall under heightened security.

Visits with inmates' attorneys are permitted but family and friend visits, which would normally start taking place Friday, will only be allowed with the approval of the warden. The lockdown is re-evaluated every 24 hours by the warden and a six-member committee, Messick explained.

While the cause of this week's fights has not been determined, Messick said staff continue to conduct interviews with inmates and thorough searches of their quarters. No serious injuries have been reported.