The plane that crashed near Athens at the weekend killing all 121 people aboard had suffered a loss of cabin pressure once before, Cyprus-based Helios Airways said on Tuesday.

No cause for the accident has been found but maintenance and pilot response are factors being examined as investigators search the mountainside where the Boeing 737-300 crashed on Sunday.

The pilot had reported a fault with the plane's air conditioning early in the flight but some experts believe it might have been slow decompression, where oxygen levels in the cabin and cockpit fall and people lose consciousness.

First autopsies on victims have suggested crew and passengers were unconscious when the plane crashed near Athens.

"We can confirm that the aircraft that experienced the decompression problem (before) was the aircraft that was involved in this accident," Helios said in a statement.

The authorities did not question the plane's maintenance following the previous incident, it said, noting the aircraft had undergone routine maintenance just last week.

The work is managed by UK-based ATC Lasham and the plane had been leased from Frankfurt-based Deutsche Structured Finance, a spokeswoman for the airline confirmed.

The fact the Boeing 737 - one of the safest planes in operation - crashed in clear skies at midday makes the incident unusual, analysts say.

Stranger still is why the pilots lost consciousness despite a separate supply of oxygen to the cockpit.

"It seems very odd that passengers would get their masks on and the pilots would not. That could point to malfunction (of the pilots' oxygen supply) or a lack of pilot training," said Henry Lupa, an aeromedical specialist who advises UK technology firm QinetiQ.

Two F-16 fighter jets scrambled to investigate the plane after it failed to make radio contact reported the co-pilot slumped in the cockpit and no sign of the pilot while their oxygen masks were seen dangling.

Reports from the crash site said some victims were frozen, suggesting possible decompression from a crack or hole in the plane which might have exposed passengers to outside temperatures of minus 50 degrees Celsius at cruising altitude.

"If the reports about the temperature drop are accurate, the chances of actual mechanical fault generating that are remote. It is more likely that the skin of the aircraft was breached," said Lupa.

Cracks in aircraft can be caused from wear as metal fatigue builds up or from an accident, including collision with refuelling or loading equipment or even birds.

First flown in late 1997, the Helios plane was young by industry standards.

In 1998 a 737 also carrying 121 people was flying from Dubrovnik, Croatia, to London when it suffered decompression.

Emergency oxygen allowed the first officer to revive the collapsed pilot and the plane descended to a level where passengers could breathe without assistance and the plane landed safely. A cracked cargo door was blamed for the incident.

The Paris-based BEA (Bureau d'Enquetes et d'Analyses) is set to examine the flight data recorder recovered from the Athens crash site on Wednesday.