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SEATTLE — U.S. investigators said Monday they have found a potential cause of a seaplane crash that killed 10 people off an island in Washington state last month.

The National Transportation Safety Board, the agency investigating the Sept. 4 crash off Whidbey Island, said it appeared a critical part that moved the planes horizontal tail stabilizer came apart, The Seattle Times reported.

That part might have failed because a clamp nut unthreaded and rotated due to a missing or improperly installed lock ring, the investigators found.

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The failure of the component, called an actuator, during flight “would result in a free-floating horizontal stabilizer, allowing it to rotate uncontrollably about its hinge, resulting in a possible loss of airplane control, the NTSB said.

The plane, a de Havilland Canada DHC-3 Otter turboprop operated by Renton-based Friday Harbor Seaplanes, crashed into Puget Sound, killing the pilot and all nine passengers. It was about half an hour into a flight to the Seattle suburb of Renton from Friday Harbor, a popular tourist destination in the San Juan Islands.

The investigators said that when the wreckage was retrieved, the upper portion of the actuator was still attached to the horizontal stabilizer while the lower portion was attached to its mount in the fuselage.

The most recent overhaul of the planes horizontal stabilizer actuator was completed April 21. The lock ring was not found with the wreckage, but several of the holes drilled in the clamp nut to accept the lock ring were damaged such that they would not allow for the full insertion of the lock ring.

At this time, the NTSB does not know whether the lock ring was installed before the airplane impacted the water or why the lock ring was not present during the airplane examination, the agency said.

The NTSB and the Transportation Safety Board of Canada have asked that the manufacturer draft instructions for all operators of DHC-3 aircraft to inspect the actuator to ensure that the lock ring is properly installed to prevent unthreading of the clamp nut.

Witnesses who saw the plane nose dive into Mutiny Bay helped officials identify the crash site. Still, it took over a week and three types of sonar to locate what remained of the plane due to its depth and the current of the channel where the aircraft hit the water.

Crews using remotely operated vessels and cranes recovered the majority of the planes wreckage from the sea floor more than 150 feet (46 meters) below the surface in late September.

The victims included a civil rights activist, a business owner, a lawyer, an engineer and the founder of a winery and his family.

Six bodies have been recovered. Those include the body of 29-year-old Gabby Hanna, which was recovered by witnesses the day of the crash, and five others found during recovery efforts.