Authorities haven't found the cause of the fire, which put at least 20 homes at risk after it began at about 5:30 a.m., the San Jose Mercury News reported Thursday.

"Our main priority is evacuations, getting people out safely. Then we can start working the perimeter of the fire," fire prevention specialist Chris Morgan said.

Fire Battalion Chief Paul Van Gerwen told the Mercury News officials are having a hard time putting out flames because of wind, timber and brush in the area.

"More than likely, the incident commanders have decided this is a fire where they are going to have to use a lot of aerial resources. They are going to have fight it this way because of the terrain," Contra Costa Fire Battalion Chief David George told KTVU-TV, Oakland, Calif.