WASHINGTON - Investigators were unable to pinpoint a cause for the Alaska plane crash that killed former Sen. Ted Stevens and four others in August 2010 but said the accident may have been the result of the pilot's "temporary unresponsiveness," the National Transportation Safety Board said Tuesday.

Although pilot Terry Smith had suffered a stroke in 2006, the reason for his unresponsiveness "could not be established from the available information," the board found.

Investigators ruled out weather conditions or mechanical problems as the reason why the plane slammed into a remote mountainside.

The board's chairwoman, Deborah A.P. Hersman, apologized for the vague finding, saying that while she was proud of the work NTSB investigators did, it was unusual and disappointing not to be able to settle on a cause.

There was no flight data recorder or cockpit voice recorder on board the single-engine de Havilland DHC-3T Otter floatplane. The front-seat passenger, one of the teenage passengers who survived, was said to be asleep at the time of the crash.

The board voted to recommend that the Federal Aviation Administration be clearer about its guidelines for issuing medical certifications to pilots who have had strokes.

Survivor James Morhard said he had expected conclusive evidence of a cause. "It's disappointing that they didn't," he said, but added that "I know they made every effort."

Four people survived the crash, including former NASA chief Sean O'Keefe. They were among eight guests at a General Communications Inc. lodge, and were flying 52 miles away to a salmon-fishing camp.

Smith, a 28-year veteran of Alaska Airlines, was considered an experienced pilot and came from a family of fliers.