Aviation experts were perplexed, saying it was rare for a plane to crash because of depressurization.

"Although there are precedents for both pilots losing consciousness at the controls of the aircraft in the past, for it to happen on a large airliner like a Boeing 737, with all the backup systems they have there, does seem to be really quite extraordinary," said Kieran Daly, editor of Air Transport Intelligence, a news agency that covers the industry.

"It really is all very peculiar," he added. "I rather suspect we're heading for a very complicated investigation."

Late on Sunday, rescue teams said they had located one of the plane's "black boxes," two orange-colored devices that record flight data and voices of the pilots.

In December 2004, three passengers were hospitalized after a Helios plane lost cabin pressure and made an emergency landing at the Larnaka airport. A year earlier, another Helios plane suffered engine problems, diverting to the Greek resort island of Rhodes.

Airliners are pressurized by a system that draws air from the engines, which compress air for internal use. A valve at the back of the plane determines how fast air is let out of the fuselage.

Aviation experts say that depressurization is rare, and that when it occurs, a warning horn sounds in the cockpit, and crew members are supposed to put on oxygen masks and descend to 10, 000 feet, where there is enough oxygen for humans to function even in a depressurized aircraft.

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Video: Passenger Jet Crashes in Greece In October 1999, a Learjet carrying the golfer Payne Stewart and five others crashed in South Dakota after flying hundreds of miles on autopilot. Air Force pilots who shadowed the jet reported that the cockpit windows were iced over, a sign of depressurization.

After the investigation of that crash, a doctor with the National Transportation Safety Board said pilots' cognitive ability could fade away quickly when oxygen was limited, often faster than they realized.

Another source of incapacitation can be smoke or fumes in the cockpit. If the problem is not obvious from the wreckage, it may be discerned from tests on the blood of the victims, which will show what they were breathing before they died.

The cockpit voice recorder is likely to capture the sound of the warning horn, if it sounded, and conversation between the pilots about the problem.

The plane that crashed Sunday was built in 1998, a year before the airline was established. Planes of that vintage commonly carry autopilots capable of climbing to a pre-assigned altitude, and following a predetermined course from point to point, without pilot intervention.

Company officials denied reports news reports from Cyprus that the plane had been withdrawn last year for technical problems.

Flight 522 left the Larnaka airport at 9 a.m. carrying 115 passengers and 6 crew members. After the plane lost contact with air traffic controllers "for more than an hour," said Ioannis Pantazaratos, head of the air traffic control tower at Eleftherios Venizelos International Airport in Athens, "we had no idea who was flying the plane."

Defense Ministry officials said two F-16 fighters were ordered to an urgent reconnaissance flight at 10:55 am. By 11:20 am, said the Greek Air Force spokesman, Yiannis Papageorgiou, "the fighter pilots had visual contact of the aircraft."

The two Greek fighter pilots who shadowed the passenger plane spotted the co-pilot slouched over the controls. He was wearing an oxygen mask, apparently unconscious, Greece's government spokesman, Theodore Roussopoulos, said after a marathon crisis meeting led by the Greek prime minister.

From then on, Mr. Papageorgiou said, "The two fighter jets escorted the passenger plane, watching it crash into the mountain at 12:03."

By Sunday afternoon, senior Greek and Cyprus officials began minimizing the likelihood of terrorism.

"The first indications," said Marios Karoyian, a spokesman for the president of Cyprus, Tassos Papadopoulos, "are that it was not caused by a terrorist act."

In Athens, a senior Defense Ministry official said: "We're not ruling out anything. But that the plane was not shot down by armed F-16 jet fighters suggests that this may not have been a terrorist attack."

Helios, Cyprus's first private carrier, was established in 1999. It flies to Dublin; Sofia, Bulgaria; Warsaw; Prague; Strasbourg, France; and several British airport