Federal Aviation Administration spokesman Ian Gregor said on a recorded telephone message that four others aboard the aircraft were critically burned in the crash on Tuesday evening about 35 miles (56 km) northwest of Redding in remote terrain.

The helicopter, with a two-person crew and 11 firefighters aboard, crashed under unknown circumstances, Gregor said.

FAA and National Transportation Safety Board investigators were on their way to the scene of the crash, Gregor added.

The four injured were first taken to Mercy Medical Center Redding, and three have been transported to UC Medical Center in Davis, California, said Michael Burke, a spokesman for the Redding hospital.