A 2010 helicopter crash in a small Idaho town that killed two state wildlife biologists and the pilot was caused when an aluminum clipboard belonging to one of the biologists somehow fell out of the aircraft and hit the tail rotor, the National Transportation Safety Board said.

A report released Thursday said it's unclear if the clipboard came from an external luggage rack or from inside the cabin.

The Hiller UH-12E helicopter had a three-abreast bench seat that had the pilot in the center. Its cabin was fully enclosed, but officials said occupants sometimes open the doors during flight.

"It's not common but pretty unusual," said lead investigator Michael Huhn.

Killed were 47-year-old Larry Barrett and 34-year-old Danielle Schiff, Idaho Department of Fish and Game biologists who were to count salmon spawning nests in the Selway River. The pilot was 43-year-old Perry J. Krinitt of Belgrade, Mont.

Fish and Game had chartered the aircraft from Clarkston, Wash.-based Leading Edge Aviation.

Officials said the pilot stowed most of the biologists' equipment in the aircraft's external racks. The helicopter then started its flight from Clarkston, with the pilot planning to fly to a fuel stop about 80 miles east.

About 33 minutes into the flight, the pilot broadcast that he was "landing in Kamiah," a small town in northern Idaho. The report said that transmission came within 4 minutes of the Aug. 31, 2010, crash.

The helicopter went down on a street in downtown Kamiah, hitting a travel trailer. No one on the ground was hurt.

The 1,500-foot debris path included parts from the tail rotor and clipboard, and the clipboard showed signs of striking the tail rotor, including paint transfer marks, the report said. Witnesses also told investigators they saw pieces falling from the aircraft before it crashed.

Investigators said they were unable to determine why the helicopter diverted to Kamiah. But they speculated the aircraft might have had a mechanical problem, or the occupants realized the clipboard was unsecured, or one of the biologists was experiencing airsickness.

Officials said it's unlikely the three needed to land to retrieve survey equipment because that could have been done during the fuel stop.

The report noted that one witness said the right cabin door was open in flight. But damage to the helicopter indicated both doors were closed at impact; investigators noted the door could have been opened and then closed during the descent.