Flying 34,000 feet over the Aegean Sea, the F-16 fighter jet pilots peered into the cockpit of the Cypriot passenger plane they were sent to save. No one was at the controls.

The co-pilot was slumped in his seat. The captain was nowhere in sight. Oxygen masks dangled in the cabin.

The fighter jets flew by a second time and saw two people apparently trying to take control of the Boeing 737.

Forty minutes later, Helios Airways flight ZU522 crashed into a mountainside near the ancient city of Marathon, breaking into pieces and bursting into flames, hurling bodies and debris across the valley.

All 121 people on board, including 48 children, were killed, making the crash the deadliest plane disaster in Greece.

The cause appeared to be technical failure -- resulting in high-altitude decompression and lack of oxygen -- and not terrorism, authorities said. A transportation official said the 115 passengers and six-member crew may have been dead when the plane went down.

"We saw some fighter jets flying very low, and after a few minutes we heard a very loud noise and saw pieces of the plane flying in the air," said Spyros Papachristou, a Grammatiko resident who saw the airliner crash at 12:05 p.m.

The plane had departed from Larnaca, in Cyprus. Family members wept as they waited at the airports there and in Athens, its destination before heading on to Prague. When relatives at Larnaca heard news of the crash, they swarmed the airline counters, shouting, "Murderers!" and, "You deserve lynching!"

A man whose cousin was a passenger told Greece's Alpha Television he received a text message on his cell phone minutes before the crash.

"He told me the pilots were unconscious. . . . He said: 'Farewell, cousin, here we're frozen,' " Sotiris Voutas said, indicating the plane was cold, a sign of decompression.

About a half-hour after takeoff at 9 a.m., pilots reported air-conditioning system problems to Cyprus air traffic control. After entering Greek air space over the Aegean, the plane lost all radio contact. Two Greek F-16 fighter jets were dispatched soon afterward.

A government spokesman, Theodoros Roussopoulos, said it was unclear whether the two people the fighter jet pilots saw in the cockpit were crew members or passengers. The plane apparently was on automatic pilot when it crashed, a Helios spokesman, Marios Konstantinidis, said in Cyprus.

"When a pilot has no communication with the control tower, the procedure dictates that other planes must accompany and help the plane land. Unfortunately, it appeared that the pilot was already dead, as was, possibly, everyone else on the plane," Cyprus Transportation Minister Haris Thrasou said.

The head of the Greek airline safety committee, Akrivos Tsolakis, said the crash was the "worst accident we've ever had." He said the plane's "black boxes," containing data and voice recordings valuable for determining the cause, had been recovered.

"There apparently was a lack of oxygen, which is usually the case when the cabin is depressurized," Tsolakis said.

The F-16 jets met the plane at 34,000 feet, the Greek air force said. At that altitude, the effects of depressurization are swift, said David Kaminski Morrow, of the British-based Air Transport Intelligence magazine.

"If the aircraft is at 30,000 feet, you don't stay conscious for long, maybe 15 to 30 seconds," he said.

This is the height of Europe's summer travel season, when Mediterranean resorts such as Cyprus are packed with tourists.

At the Greek crash scene, more than 100 firefighters, backed by planes and helicopters dropping water, fought a brush fire caused by the crash.

Fire department rescue vehicles carried body bags up the steep slopes of the charred valley to a fleet of ambulances. Black-robed Greek Orthodox priests were on hand.

Relatives from Cyprus were to be taken to a reception center near the Athens airport, but the remains of many victims were charred beyond identification.

The Cyprus transportation minister said DNA tests would be necessary.

Rescuers work on a hillside near the tail of the Cypriot Helios Airways jet that crashed on its way from Larnaca, Cyprus, to Athens International Airport.A mother cries while waiting at Larnaca International Airport for news of her loved ones, who were on flight ZU522. Forty-eight children were aboard the jet.One of the engines of the Helios Airways jet that crashed near the coastal town of Grammatiko, Greece. In rear, a firefighting plane flies overhead to put out a blaze that was caused by the wreck, the deadliest ever in Greece.