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ESTES PARK, Colo. (AP) _ A helicopter crashed and killed its pilot while fighting a forest fire because its turbine engine overheated and partially melted, investigators said.

National Transportation Safety Board investigators were still trying to determine what caused the overheating.

The helicopter crashed July 30 near Rocky Mountain National Park, the second fatal crash at that fire.

``Witnesses reported hearing a high-pitch whine and seeing rotor blades turning slowly, so we suspected a problem with the power train,'' said Arnold Scott, chief investigator for the Denver office of the NTSB.

Investigators took the engine apart and found that part of it ``is just literally toast, just burned up,'' Scott said Monday.

The damage interrupted the transfer of power to the helicopter's rotor blades, he said.

``It's like going down the highway at 60 miles an hour in a standard shift car and, without taking your foot off the accelerator, pushing in on the clutch. Your engine overspeeds but you have no power being developed,'' he said.

Federal Aviation Administration investigators examined maintenance records for the Aerospatiale SA-315B Lama helicopter, which was owned by Geo-Seis of Fort Collins. The rebuilt engine on the 1978 helicopter had logged only about 200 hours of flying time.

The crash killed the pilot, Leonard G. Knight, 52, of Boulder, the only person on board.

He was the 15th person killed this summer while fighting or en route to fighting wildfires that have burned more than 4 million acres.