The helicopter had lifted off from a clearing in a remote, rugged region of the Shasta-Trinity National Forest, said Jennifer Rabuck, spokeswoman for the U.S. Forest Service.
The aircraft was carrying 11 firefighters and two crew members when it went down Tuesday night in the Shasta-Trinity National Forest, according to the Federal Aviation Administration and the National Transportation Safety Board.
Four people were flown to hospitals with severe burns, according to the Forest Service.
Two of the survivors were in critical condition at the University of California Medical Center in Sacramento, Forest Service spokesman Mike Odle said Wednesday, according to CBS station KPIX. The other two survivors were in serious condition at Mercy Medical Center in Redding, he said.
The Sikorsky S-61N chopper was destroyed by fire after crashing "under unknown circumstances," said FAA spokesman Ian Gregor. The NTSB was leading the investigation and was headed to the scene, about 215 miles northwest of Sacramento.
Firefighters who were waiting to be picked up helped rescue the injured after the helicopter crashed around 7:45 p.m. and caught fire, Rabuck said. About three dozen firefighters had to spend the night on the mountain because it became too dark for other helicopters to land, she said.
Nine people - a co-pilot and eight firefighters - were still missing in the wreckage and presumed killed. Recovery efforts have been complicated by the remote location, and the wreckage is still burning, Rabuck said.
"It's difficult to access," she said. "It's very remote, very steep and heavily forested."
The firefighters had been working at the north end of a more than 27-square-mile fire burning in the Shasta-Trinity forest, part of a larger complex of blazes that total 135 square miles. The complex of fires in that forest were about 87 percent contained, according to KPIX.
"We are praying for the swift recovery of all the victims, and our hearts go out to their loved ones," Gov. Arnold Schwarzenegger said Wednesday.
Some of the firefighters, including those in the hospital, were employed by firefighting contractor Grayback Forestry, based in Merlin, Ore. Mike Wheelock, Grayback's founder and owner, would not confirm any deaths.
Grayback firefighters Michael Brown, 20, and Jonathan Frohreich, 18, as well as a co-pilot of the helicopter, were being treated at the University of California, Davis Medical Center in Sacramento, according to the contractor.
Brown was upgraded to fair condition late Wednesday, Frohreich remained in critical condition and the co-pilot was in serious condition, according to the hospital and fire officials.
Another Grayback employee, identified as Rick Schoeder, 42, was in serious condition at Mercy Medical Center in Redding, officials said.
The helicopter was owned and operated by Carson Helicopters Inc., a Pennsylvania company whose firefighting operations are based in Grants Pass, Ore. All 12 of the company's helicopters are being used for firefighting in Oregon and California, said Bob Madden, Carson's director of corporate affairs.
The helicopter's two co-pilots were Carson employees, Madden said; one was hospitalized and the other was among the missing.
Meanwhile, fire crews worked to contain a series of small fires sparked by a storm that generated an estimated 2,000 lightning strikes in California, Oregon and Nevada over the past two days, said Basil Newmerzhycky, a Forest Service meteorologist.
None had grown into major blazes so far, Newmerzhycky said. By contrast, a storm on June 21 generated about 8,000 strikes that sparked more 2,000 fires, becoming the largest fire event in California history.
A wildfire outside Yosemite National Park that was started July 25 by a person taking target shooting practice was fully contained by Wednesday morning after destroying 30 homes and consuming about 53 square miles.
Before Tuesday's helicopter crash, three firefighters had been killed while on duty in California this year, including one firefighter also assigned to battle the Shasta-Trinity blazes who was killed last month by a falling tree.
On July 2, a volunteer firefighter in Mendocino County died of a heart attack on the fire line. Another firefighter was killed July 26 in when he was burned while scouting a fire.