YAKIMA, Wash. -- The pilot in a 2007 crash near White Pass that killed him and nine skydivers was likely impaired by a lack of oxygen when the plane stalled out, according to the National Transportation Safety Board.

Philip Kibler, 46, had been flying for more than an hour above 14,000 feet, where substantial hypoxia is likely to occur, according to the agency's final report on the probable cause of the crash.

Hypoxia, a condition caused by a lack of oxygen, limits the ability to perform complex tasks. The problem can start at about 8,000 feet and becomes more intense above 12,000 feet.

Because the pilot was likely impaired, he probably would not have been aware of the symptoms, the board found.

The plane -- a Cessna 208-B commonly used for skydiving drops -- went through a sequence of full turns while climbing and descending -- an apparent attempt to locate clear flying conditions.

The pilot had checked the weather and delayed the flight until he thought the weather would be clear, investigators said.

However, the crash occurred on a mostly dark night, and the flight likely encountered clouds, turbulence and possibly icing in the last three minutes of flight, according to the ruling.

Investigators earlier suggested that icing had not played a significant part in the crash.

Ice had been considered as a possible factor because pilots have had problems flying that model in icing conditions.

The pilot was rated for instrument flight, but company policy was to fly in visual conditions only, according to the NTSB, which issued its ruling late last month.

The pilot and nine skydivers, based in Snohomish, Wash., were returning from a weekend skydiving event in Idaho and were en route to Shelton, Wash.

The families of the skydivers have sued Cessna in federal court, alleging the company knew that the plane was dangerous in icing conditions.

Several lawsuits against Cessna allege that at least some of the 208-B models involved in crashes had improperly calibrated stall-warning equipment when they left the factory.

Cessna has argued that the calibration issue had nothing to do with the crashes and that calibration routinely must be checked regardless. The company claims in court filings that the plaintiffs are improperly relying on FAA airworthiness directives to make their case.

The lawsuits, which are being heard together by a Kansas judge, await trial.