Washington (CNN) -- Federal accident investigators admitted Tuesday they don't really know what caused the deadly crash in Alaska last year of a plane carrying former U.S. Sen. Ted Stevens. The longtime Republican was among five people killed, including the pilot, who was taking a group to a fishing camp in the rugged Alaskan terrain.
The best guess from the National Transportation Safety Board points to 62-year-old Terry Smith, an experienced pilot who knew the area well.
In a report Tuesday, NTSB investigators concluded that "the probable cause of this accident was the pilot's temporary unresponsiveness for reasons that could not be established."
Circumstantial evidence was not conclusive, but it ruled out mechanical failure of the aircraft. Instead, the pilot's medical history, and observations by three witnesses in the weeks ahead of the fatal flight, suggested something happened with the pilot's ability to fly the plane.
"What we do not know -- and may never know -- is what happened in the last three minutes of that fatal flight," NTSB chairman Deborah Hersman said at a hearing, where staff presented the findings of a nine-month investigation.
Hersman said, "We know that something happened in that cockpit" and that Smith had been recertified in 2008 after suffering a stroke two years earlier. What could not be explained was why he pulled up and veered left, crashing directly into a ridge.
About a month before the crash, Smith was at a fundraising event in Alaska, offering sightseeing rides to passengers, when he failed to move toward takeoff when it was his turn. Witnesses said he was simply staring ahead and not communicating. Eventually, he piloted the plane to takeoff and a safe return.
NTSB medical experts said autopsies found no evidence of any medical problems that could have caused the pilot to crash on the Stevens flight on August 10, 2010. There were no flight recorders on the old plane, and passengers who survived said they do not remember anything going wrong before impact.
The report found plenty of areas where things could have been done differently, including that the survivors of the crash didn't know they had a way to immediately call for help. Instead, it took hours to discover the plane had crashed and hours more to find and reach the wreckage and rescue the four survivors.
The reasons for the delay: An emergency crash beacon was not mounted properly on the single-engine pontoon aircraft, a 1957 de Havilland, and the unit broke away from its antenna on impact. The locating device triggered as designed, but without an antenna, the signal from the transmitter could not reach rescue teams, investigators said.
The pilot, Smith, carried a portable satellite phone on board, but apparently failed to tell any of the passengers. The phone was found in the cockpit, working and within reach of the survivors, in an unmarked black plastic carrying case.
"The delay in accident notification did not result in additional fatalities," the report concluded, suggesting that Stevens, the pilot and three other passengers were mortally injured at the time of the crash.
The NTSB reviewed and recommended changes in medical certification of pilots who've suffered a stroke, to require a detailed analysis of their cognitive abilities. In the Stevens crash, the pilot was recertified to fly just two years after his stroke, a timetable that has little basis in determining fitness to fly.
Among the findings, the report said "the Federal Aviation Administration's internal guidance for medical certification of pilots following stroke is inadequate because it is conflicting and unclear, does not specifically address the risk of recurrence associated with such an event, and does not specifically recommend a neuropsychological evaluation (formal cognitive testing) to evaluate potential subtle cognitive impairment."
The longest-serving Republican senator in U.S. history, Stevens lost his 2008 re-election bid to then-Anchorage Mayor Mark Begich after being convicted on charges of making false statements on financial disclosure forms.
A federal judge later dismissed the conviction, citing prosecutorial misconduct.
Stevens survived a plane crash that killed his wife in 1978; he later remarried.