(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.
This Wednesday image shows the crash site, according to GPS coordinates given by the U.S. Forest Service.
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.
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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.
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On Tuesday, a Sikorsky S-61  helicopter crashed northwest of Redding, California. The helicopter had been carrying 11 firemen who had been battling a wildfire just north of Junction City, and two crewmen. Eight of the firefighters and one of the crewmen are missing and presumed dead.  
A Sikorsky S-61 similar to the one involved in the crash. 
Four out of the 13 members of the flight were rescued from the crash, though they received severe burns. Sharon Heywood, a United States Forest Service spokesperson said that the three firefighters and pilot had been initially airlifted to Mercy Medical Center in Redding. However, because of the critical condition of two of the firemen and the pilot, the three of them were transferred to UC Davis Medical Center in Sacramento. The remaining firefighter remains at Mercy Medical Center in serious condition.

The helicopter, which is owned by Carson Helicopters, crashed "under unknown circumstances," according to Ian Gregor, a spokesman for the Federal Aviation Administration (FAA). The helicopter crashed 35 miles northwest of Redding in the Trinity Alp region, at roughly 7:30 p.m. Tuesday evening, said Gregor. Both the FAA and National Transportation Safety Board investigators were on the scene of the crash.

The firefighters had been fighting flames covering more than 27 square miles in the Shasta-Trinity National Forest. They had been working near the north end of the fire, which is reportedly 87% contained.