(CNN) -- Nine people were missing and presumed dead and four were hospitalized Wednesday after a helicopter carrying firefighters crashed the night before in northern California, aviation spokesmen said.
The Sikorsky S-61 helicopter entered a remote area to pick up firefighters battling wildfires, said Ian Gregor of the Federal Aviation Administration. The crash happened about 7:30 p.m. Tuesday, 35 miles northwest of Redding in the Trinity Alps Wilderness area of the Shasta-Trinity National Forest.
Two crew members and 11 firefighters were aboard, Gregor said. Three of the injured were contract firefighters, and the fourth was the pilot, said Sharon Heywood, the forest's supervisor.
All of the injured initially were taken to Mercy Hospital near Davis, California. A firefighter in critical condition and two people in serious condition -- a firefighter and the pilot -- were transferred to the University of California Davis Medical Center, Heywood said.
The third firefighter remained at Mercy in serious condition Wednesday, she said.
Heywood said the crash site was difficult to access because of its terrain.
U.S. Forest Service spokeswoman Daria Day said the helicopter, assigned to a group of four fires burning in the forest, went down near the north end of the so-called Buckhorn fire. She said she didn't know if the helicopter was landing or taking off.
The helicopter was operated by Carson Helicopters, which has offices in Grants Pass, Oregon, and Perkasie, Pennsylvania, according to Sacramento, California, TV station KCRA.
"We don't know what occurred," Carson Helicopters' Bob Madden told KCRA. "We haven't examined the aircraft yet. We've never had a copter go down due to firefighting efforts."
The crash was being investigated by the FAA and the National Transportation Safety Board, Gregor said.
The Buckhorn fire -- part of the Iron Complex fires in the forest -- has burned more than 18,500 acres since it started June 21, the Forest Service said. It was 25 percent contained Wednesday, according to the service.
The four Iron Complex fires have burned more than 84,000 acres since they started on or about June 21, the service said.