Tests on victims of the Helios Airways Boeing 737 crash showed no carbon monoxide poisoning, which appeared to rule out one theory for the disaster.

Investigators hope tests will shed some light on why the pilot, co-pilot and many passengers on the Helios 737 apparently fell unconscious before the plane crashed near Athens last Sunday killing all six crew and 115 passengers.

One theory for Greece and Cyprus's worst air disaster was faulty air conditioning or a fire releasing poisonous carbon monoxide fumes.

"We are still doing tests for other gases, poisons, drugs and alcohol," Greece's Chief Coroner Philippos Koutsaftis said after meeting Justice Minister Anastasios Papaligouras.

Of six victims examined, five, including the co-pilot, showed no signs of breathing in carbon monoxide, while a stewardess had a minimum level of 7 percent, which was not considered dangerous, Koutsaftis told reporters.

"The seven percent is deemed to be minimal," he said.

Former Cypriot government forensic pathologist Marios Matsakis said there had been insufficient tests so far to draw any firm conclusions.

"All we can say at this point is that they did not breathe in carbon monoxide," Matsakis, a member of the European Parliament, said.

At the crash site, searchers found the cockpit voice recorder after six days of scouring the area, raising hopes the last conversations would shed light on the disaster. The plane's other so-called "black box", which records flight data, had already been found.

Cyprus President Tassos Papadopoulos has described the crash as a "peculiar" accident with only one precedent, suggesting a link to US golfer Payne Stewart's death in 1999.

Stewart was not the pilot of a Learjet that crashed in the United States, killing all five people on board.

In both cases, the planes flew for a lengthy period on auto pilot -- Stewart's for four hours halfway across the United States -- with the pilots in both cases seemingly unconscious or dead at the controls and out of radio contact.

The US investigation of Stewart's October 1999 accident in which all five people aboard died, warned of dangers of pilots becoming so engrossed in a minor task they did not swiftly put on oxygen masks when trouble broke out.

Investigators believe the key to discovering what caused the Cypriot plane to crash may be found in the final 23 minutes of the flight scheduled to last only 90 minutes from Larnaca in Cyprus to Prague with a stop in Athens.

F-16 fighter pilots whose planes were scrambled to fly alongside the doomed aircraft said they could not see the pilot, the co-pilot was visible slumped in his seat unconscious, and two unknown people were in the cockpit apparently trying to fly the plane.