MI6 spy Gareth Williams would have suffocated within three minutes after getting inside his sports holdall, an inquest heard today.
Poisoning and asphyxiation are the "foremost contenders" in solving the death riddle, pathologists said.
It also emerged scientists found traces of "at least" two unknown people in his upmarket London apartment despite evidence Mr Williams rarely invited people over.
Forensic expert Ros Hammond said there were hopes of a breakthrough "within a matter of weeks" from DNA tests on a green towel discovered in his kitchen.
"There's hope," she told Westminster Coroner's Court. "The tests are still in progress and there may be some promising results from those tests."
Hammond said a third party would not necessarily have left any DNA on his red North Face bag and padlock.
But she added: "There's certainly evidence of at least two people other than Mr Williams on the samples tested."
The 31-year-old was probably suffocated or killed by a poison which disappeared in his system during decomposition, pathologist Benjamin Swift said.
Dr Swift said his post-mortem examination was hampered by levels of heat within the bag after radiators were mysteriously turned on in Mr Williams's top-floor flat in the middle of summer.
Ian Calder, who performed the second post-mortem examination, observed that the build-up of carbon dioxide would have become poisonous to Mr Williams within about two or three minutes, had he been alive when he entered the bag.
"The toxic effect of the carbon dioxide... plays some considerable havoc with the chemistry of the body and so as a result of that the accumulation of carbon dioxide has quite a considerable effect on the wellness of the individual," he said.
The results would include headaches, then confusion and eventually unconsciousness and cardiac arrest, he added.
A soporific state would have been induced before the spy lost consciousness, which could have prevented him from trying to escape, he explained.
"He would get into a situation of not being aware, of not being able to react to getting himself out of the environment," Dr Calder said.
Hypercapnia - a high level of carbon dioxide in the blood - would be a "reasoned explanation" of what might have happened to Mr Williams, the pathologist noted.
"I think it's a very likely possibility considering we have a healthy person with no damage, as far as we know no drugs, no trauma, no natural disease," he said.
Examinations on August 25 2010 - two days after Mr Williams was found in a holdall in his bathroom - gave cause of death as "unascertained".
But under questioning today, Dr Swift said poisoning or asphyxiation such as suffocation were "probably rather than possibly" to blame.
Dr Swift said the two causes of death "were certainly two of the more prominent" beliefs as he conducted examinations.
When family lawyer Anthony O'Toole asked if there were any other possible causes of death, Dr Swift replied: "I would never say never but those are the foremost contenders."
Another pathologist, Richard Shepherd, also said it was "more likely (Mr Williams) was alive when he entered the bag than that he was dead".
There was, however, "no suggestion" the spy's body had been manhandled into the holdall and were he to have been forced into it either alive or straight after he died, marks on his body would have been expected, Mr Shepherd said.
Dr Swift said he believed Mr Williams would have died shortly after his last-known movements on August 15 in his top-floor apartment in Pimlico, central London.
The evidence came after bag experts said even Harry Houdini would have struggled to lock himself in the bag.
The inquest was adjourned until tomorrow. Coroner Fiona Wilcox is likely to deliver her verdict on Wednesday.