WEAVERVILLE, Calif. -- Officials investigating a deadly helicopter crash in Northern California say the cockpit voice recorder suffered heat damage that's complicating efforts to extract information from the device.

Nine people were killed and four people injured when the firefighting aircraft went down in the remote Shasta-Trinity National Forest last week.

The National Transportation Safety Board said Friday the recorder will be sent next week to England, where the device's British manufacturer will help investigators try to recover recordings that could shed light on what caused the crash.

The agency says a team of investigators is in Portland, Oregon, this week to examine the chopper's two engines, which were taken there after being recovered from the wreckage.

The helicopter was ferrying 10 firefighters, two pilots and a U.S. Forest Service employee back to base camp after crews battled a fire about 35 miles northwest of Redding.

The Sikorsky S-61N helicopter had just been refueled when it lifted off from a remote clearing, struck a tree and plummeted into a hillside, according to NTSB officials. The chopper then erupted into flames.

Two of the four men who survived the crash, firefighters Michael Brown, 20, and Jonathan Frohreich, 18, both of Medford, Ore., have been discharged from the UC Davis Medical Center in Sacramento. They suffered facial burns and broken bones.

Firefighter Rick Schroeder, 42, was also released from the hospital after suffering a cracked shoulder and vertebra along with scratches, bruises and a badly cut lip.

Pilot William Coultas, 44, of Cave Junction, Ore., has undergone skin grafting for severe burns. He was in critical condition Sunday, said Martha Alcott, a spokeswoman for the medical center.