Correction: Earlier versions of this article gave an incorrect first name for one of the National Transportation Safety Board investigators. He is Mitchell A. Garber, not Michael. This version has been corrected.

A month before he flew a plane carrying former U.S. senator Ted Stevens into an Alaskan mountain last summer, pilot Terry Smith seemed so unfocused on one occasion that an aviation museum staff member worried that he was unfit to fly their planes, according to testimony Tuesday before the National Transportation Safety Board.

“There’s something wrong with Terry,” the staff member said, according to the testimony. The employee reported that he found the veteran Alaskan pilot “staring into space” like an “Alzheimer's patient” as he was about to fly a plane at the Alaska museum.

Witnesses also testified about concerns that a 2006 stroke might have affected Smith’s cognitive abilities.

At the end of the hearing, however, the NTSB was unable to determine what caused Smith to go off course and fly into the side of mountain last summer, killing Stevens (R-Alaska) and four others.

“We just didn’t have enough weight behind any of the possible scenarios to draw a conclusion,” said NTSB Chairwoman Deborah A.P. Hersman.

The plane crashed into a remote, rugged mountainside Aug. 9 while flying Stevens and a party of friends and colleagues from a log-cabin lodge on Lake Nerka to a camp where the silver salmon were running.

Smith, Stevens, Washington lobbyist William “Bill” Phillips Sr. and an executive with the telecommunications company that owned the lodge and her teenage daughter died in the crash. All four of the survivors — former Stevens staff members Sean O’Keefe and Jim Morhard, O’Keefe’s college-age son, Kevin, and Phillips’s 13-year-old son Willy — were seriously injured.

According to the testimony presented at the two-hour hearing, Smith was alert and conversant during lunch prior to the flight, took off with eight passengers on board and flew a normal path on a familiar route and then inexplicably turned into the mountainside.

Alarms went off in the cockpit about five seconds before impact, and Smith tried without success to put the plane into a steep climb. The sound on a system that would have given an alarm 30 seconds before the crash had been turned down.

Three autopsies on Smith failed to provide any evidence he had suffered a stroke or a seizure. But investigators did not rule out either possibility.

Kevin O’Keefe was asleep in the co-pilot’s seat, and none of the other survivors was positioned to observe Smith in the minutes before the crash.

Mitchell A. Garber, an NTSB investigator, said Smith had suffered from disorientation and confusion after suffering a stroke at age 58.

“He had to relearn certain skills having to do with driving and flying,” Garber said.

Investigators said Smith also had experienced three “life events” in the weeks leading up to the crash. He had retired for the second time after a long career in commercial aviation in Alaska, his daughter had married, and a son-in-law had been killed in an airplane accident.

Citing reports that stress might have resulted in fatigue, Garber said, “We do know that sleep deprivation can increase the risk of a seizure.”

The NTSB was critical of the process by which the Federal Aviation Administration cleared Smith to return to flying after his 2006 stroke. NTSB staff investigator Mitchell A. Garber told the board that the FAA restored Smith’s license to pilot commercial planes without fully evaluating whether he was fit to fly.

He said the FAA relied solely on the word of a flight surgeon who was “out of his element” in evaluating a patient who FAA officials knew had cognitive problems for months after the stroke.

FAA spokesman Les Dorr said the agency has revised procedures to require more stringent evaluation of pilots who suffer strokes.

“We’re confident that since we’ve changed the policy no post-stroke pilots have been certified,” Dorr said.

Hersman also was critical of the FAA for declining to follow an NTSB recommendation to require inexpensive on-board video recorders on small planes.

“Technology is moving faster than the FAA moves,” she said.