The Cypriot (search) airliner that crashed Sunday in Greece, killing all 121 people on board, may have experienced a catastrophic loss of cabin pressure that rapidly starved the pilots of oxygen, aviation experts said.

At 34,000 feet, the co-pilot of the Helios Airways (search) flight from Larnaca, Cyprus, was slumped over the controls and the captain was not in the cockpit, said the Greek air force, which sent two F-16 jets to intercept the airliner after it lost radio contact. Oxygen masks dangled inside in the cabin of the Boeing 737 (search), a Greek government spokesman said.

"If the aircraft lost cabin pressure, either in the cockpit or the cabin, effectively everybody would be doomed within a short space of time," said Chris Yates, an aviation analyst at Jane's Transport.

"If the aircraft went into depressurization very quickly, you only have a few seconds before you lose consciousness, because you don't have any air to breathe at that altitude," Yates said.

Airplane cabins are pressurized so the air remains breathable at very high altitudes. If a plane suffers a loss of air pressure, the crew typically has enough warning to try to get to a lower altitude.

"It is one of these things that can be serious ... but if it occurs, there should be an automatic deployment of oxygen masks," said David Kaminski Morrow, deputy news editor of the British-based magazine Air Transport Intelligence.

"The crew get oxygen, passengers will be on oxygen, and there is enough in the tanks to get the aircraft to an altitude where you don't need the oxygen masks anymore," he added.

The crew would attempt to bring the plane down to an altitude of about 10,000 feet, where the air is breathable. This would typically be done over the sea rather than a mountainous area, Kaminski Morrow said.

"If the aircraft is at 30,000 feet you don't stay conscious for long, maybe 15 to 30 seconds. It is like standing on top of Mount Everest," Kaminski Morrow said.

Captain Denis Breslin, spokesman with the U.S. Allied Pilots Association, said that in Boeing planes, a warning horn and light notify pilots of a drop in pressure. "It's very loud and very noticeable."

He said American pilots are trained to take advantage of the "period of useful consciousness" — no less than 10 to 15 seconds — giving pilots enough time to put on their "quick don" oxygen masks, which cover the nose and the mouth.

A lack of oxygen apparently caused the 1999 crash that killed golfer Payne Stewart and four others aboard Stewart's twin-engine jet. The plane went down in a pasture in South Dakota after flying halfway across the United States on autopilot, as the five apparently became unconscious after the plane lost cabin pressure.

In this case, however, experts were puzzled by reports that passengers may have remained conscious longer than the pilots.

One passenger reportedly sent a text message before the crash to a relative, saying the pilots were unconscious and "here we're frozen" — a reference to cold temperatures in the plane, indicating decompression.

Greek government spokesman Theodoros Roussopoulos said that when the F-16 pilots flew by the Helios Airways plane a second time, they saw two people apparently trying to take control of the Boeing 737. It was unclear whether they were passengers or pilots.

"The cabin pressure and the cockpit pressure are the same," said Yates, the Jane's analyst, noting that a failure should affect both equally.

"It's possible some of the passengers on board, because of their age, because of their health or whatever, may have been able to last a bit longer," he said.