Carbon monoxide did not knock out some of the passengers and crew of a Cypriot airliner before it crashed in the Greek mountains, coroners said Friday, deepening the mystery as to what caused the disaster that killed all 121 people on board.

Chief Athens coroner Fillipos Koutsaftis (search) said tests were carried out on the remains of the co-pilot, three female flight attendants, an infant and an adult who were on the flight that went down Sunday about 25 miles north of the Greek capital.

More tests were being conducted to determine what could have rendered crew and passengers aboard the Helios Airways (search) Flight ZU522 from Larnaca, Cyprus, unconscious during the flight. The plane flew on autopilot before crashing.

Koutsaftis said a few more days were needed to run the toxicological tests.

"This was the fastest test and the most secure," he said after meeting Justice Minister Anastasios Papaligouras (search). "We are still doing tests for other gases, poisons, drugs and alcohol."

Earlier tests showed that at least 26 people on the flight were still alive when it crashed. There has been speculation that an electrical fire or some other cause could have flooded the cabin with carbon monoxide or another gas.

"I don't know and can't rule out" that something else could have been responsible, Koutsaftis said.

Investigators also have been examining whether a sudden decompression sucked oxygen out of the cabin and cockpit, which also would have knocked out those on board.

Greece's deputy fire chief, Andreas Kois, said that missing parts of a voice recorder were found near the burned wreckage. Investigators already have sent one of the "black boxes" — the flight data recorder — to France for analysis.

A team of six coroners was conducting toxicology tests on some of the 118 bodies recovered from the crash site. Three bodies are missing.

The coroners said they expected test results to indicate whether those on board the plane had inhaled carbon monoxide.

Autopsy results on 26 bodies identified so far have shown that some passengers and at least four crew members — including the co-pilot — were alive, but not necessarily conscious, when the plane went down.

Chief investigator Akrivos Tsolakis told The Associated Press on Thursday that an air traffic control diagram showed the plane flew on automatic pilot to Athens' international airport. But it was flying at 34,000 feet and turned south into a holding pattern over the island of Kea after passing over the airport. More than an hour later, it changed course again and later crashed north of Athens.

"What troubles us is that the automatic pilot was functioning up to a certain point, and then it was disengaged, possibly by human action," Tsolakis said.

The automatic pilot had been programmed to fly the plane to Athens airport, and it was unclear how or why it was disengaged, Tsolakis said.

"Possibly, there was human intervention. I'm not speaking with certainty, because I don't have all the evidence yet," Tsolakis stressed.

The strange circumstances of the flight — and disturbing scenes witnessed by Greek air force F-16 fighter pilots scrambled to intercept the plane — have baffled authorities.

On Wednesday, state-run and private media, quoting anonymous defense ministry officials, said the two fighter pilots saw someone in the cockpit take control of the plane, which was flying in a gradually descending holding pattern apparently on autopilot.

That person, probably a man who experts say must have had flight training, then banked the plane away from Athens, lowering it to 2,000 feet and then climbing back up to 7,000 feet before the plane apparently ran out of fuel and crashed.