San Francisco ? Eight firefighters and a pilot are presumed dead in the crash of a helicopter that had just picked up workers battling a blaze in a Northern California forest, officials said Wednesday.

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, including two in critical condition, according to the Forest Service.

The Sikorsky S-61N chopper was destroyed by fire after crashing “under unknown circumstances,” said FAA spokesman Ian Gregor.