The 1999 crash of a Learjet 35 carrying golfer Payne Stewart (search) after it flew halfway across the country on autopilot may help investigators understand what happened to the Cypriot airliner that crashed Sunday into a hillside in suburban Athens.

The National Transportation Safety Board (search), which investigated the Stewart crash, concluded that crew members were incapacitated because they didn't obtain oxygen when the cabin lost pressure.

Jim Hall, who was NTSB chairman at the time, likened the crash of the Learjet 35 (search) to the crash of the Helios Airways Boeing 737-300 (search) that killed all 121 aboard.

"It's very similar to the Payne Stewart crash," Hall said.

Early reports indicated the Boeing jetliner suddenly decompressed at a high altitude. Temperatures and oxygen levels would have plummeted and left everyone aboard unconscious and freezing as the plane flew on autopilot long before it crashed, experts said Monday.

Hall said investigators will want to know whether the pilots put their oxygen masks on quickly enough and whether the oxygen in the cockpit failed.

"The accident did not have to occur," Hall said. "It has to be either a training or an equipment issue."

The NTSB's Robert Benzon, who headed the probe into the Stewart crash, led a team of three other NTSB investigators who arrived in Athens on Monday to help find out what caused the Helios Airways accident, the worst in Greek history.

If there was a sudden loss of cabin pressure, aviation experts said they couldn't understand why the pilots and flight attendants didn't react the way they were trained to.

"It's odd," said Terry McVenes, executive air safety chairman for the Air Line Pilots Association (search), International. "It's a very rare event to even have a pressurization problem, and in general crews are very well trained to deal with it."

When the aircraft flew into Greek airspace, Greek air traffic controllers couldn't raise the pilots on the radio and fighter jets intercepted the plane, flying at 34,000 feet.

The fighter pilots saw that the airline pilot wasn't in the cockpit, the co-pilot was slumped over his seat and oxygen masks dangled, government spokesman Theodoros Roussopoulos said. He said the air force pilots also saw two people possibly trying to take control of the plane.

It is that sequence of events that puzzles aviation experts.

Warnings should go off if an airliner suddenly loses pressure, and pilots are trained to immediately put their oxygen masks on and dive to about 12,000 feet, where there's enough oxygen for people to breathe, they say.

Paul Czysz, emeritus professor of aerospace engineering at St. Louis University, questions the decompression theory because people apparently were trying to fly the plane and the co-pilot was slumped over.

"He couldn't have been unconscious for a small decompression at 34,000 feet," Czysz said. "Something's amiss."

The pilot and the co-pilot would have had far more oxygen than the passengers, who have about 15 minutes, he said.

The chief Athens coroner, though, said at least six of the victims were alive when the plane plunged into the ground. But he couldn't determine whether they were conscious.

The fighter pilots also didn't report any windows out or holes in the fuselage, the most likely causes of a catastrophic loss of pressure, said Bill Waldock (search), an aviation safety professor at Embry-Riddle Aeronautical University in Arizona.

Another clue to a sudden pressure loss would have been frost on the windows because it's so cold at 34,000 feet, said Waldock.

If the fighter pilots could see into the cockpit, the windows couldn't have been iced over, as they were in the Learjet 35 crash that killed Stewart, he said.

John Nance (search), a pilot and broadcast aviation analyst, noted that the Stewart accident prompted the NTSB to warn about a problem that could have caused the Helios crash.

"They were discovering a disturbing number of times in which the crew got myopically focused on trying to solve a deteriorating problem and reacted too slowly in putting on their oxygen masks," Nance said. "A minor problem becomes a blowout."

The Helios pilots may have been trying to fix the air conditioning — a problem they'd reported in Cyprus airspace — and gotten quickly disoriented without sufficient oxygen, he said.

"You just drift off, and you don't know anything is wrong," Nance said.

Another possibility, he said, was that the co-pilot wasn't getting any oxygen and the pilot crawled behind his seat to try to fix the valve. That's why the fighter pilots wouldn't have seen him in his seat, Nance said.