Four people were taken to hospitals with severe burns, including two in critical condition, according to the U.S. Forest Service.
The Sikorsky S-61 chopper was destroyed by fire after crashing "under unknown circumstances" in a remote mountain location, Gregor said. FAA and National Transportation Safety Board investigators were headed to the scene, about 215 miles northwest of Sacramento.
The nine were presumably killed in the fire that destroyed the helicopter, Gregor said.
The firefighters had been working at the northern end of a fire burning on more than 27 square miles in the national forest, part of a larger complex of blazes that is mostly contained.
Two of the injured were flown in critical condition to the University of California, Davis Medical Center in Sacramento, Forest Service spokesman Mike Odle said Wednesday. The other two were taken to Mercy Medical Center in Redding in serious condition, he said.
Another firefighter assigned to battle the same series of wildfires died late last month when he was hit by a falling tree.