Federal investigators found a broken mechanical part yesterday in the engine of the helicopter that plunged into the Washington Channel early Friday, a spokesman said.

Laboratory tests will be conducted on the part, a piece of the drive-train that powered the Bell Jet Ranger helicopter's main rotor, to determine how and when it was broken, said National Transportation Safety Board spokesman Michael Benson.

The safety board has not determined the cause of the 7:30 a.m. crash, which killed three passengers and injured the helicopter's pilot.

The pilot, Jack C. Turley, told investigators that the Allison C20 turbine engine failed just before the aircraft dropped from an altitude of 200 feet into the water between East Potomac Park and Water Street wharf.

Several witnesses described hearing the engine stop before the aircraft splashed into the water and overturned.

The three passengers, free-lance photographer William S. Weems, 44, of Northwest, Robert Joy, 45, of Northwest, and Victoria Hinckley, 24, of Alexandria, died of injuries caused by the impact and submersion in the water for at least 20 minutes. Turley, 37, of Baltimore escaped from the helicopter and was in fair condition yesterday at George Washington University Medical Center.

The cabin area of the helicopter was crushed by the impact, but the engine, main and tail rotors, and transmission suffered little damage.

Safety board officials shipped the engine to the Allison Gas Turbine manufacturing plant in Indianapolis and unsuccessfully tried to start it, Benson said.

The investigation team then took the engine apart, and found the broken piece, he said.

The safety board will make no statement on the probable cause of the accident until after it completes its investigation, which will include tests on the broken part, the rest of the engine and other parts of the helicopter, Benson said.