MI6 spy Gareth Williams was either dead or unconscious when he was placed in the sports holdall in which he was found dead, an inquest heard today.
An expert said even world-famous escapologist Harry Houdini "would have struggled" to squeeze himself into the bag.
Peter Faulding said he believed a third party was present, describing theories that Mr Williams got inside the holdall by himself as "unbelievable scenarios".
Police discovered the naked decomposing body of the 31-year-old spy padlocked inside a red North Face holdall in the bath of his flat in Pimlico, London, on August 23 2010.
Mr Faulding, a former Parachute Regiment reservist who specialises in rescuing people from confined spaces, made 300 unsuccessful attempts to lock himself inside an identical 81cm x 48cm bag.
"I couldn't say it's impossible, but I think even Houdini would have struggled with this one," he said.
The expert added: "My conclusion is that Mr Williams was either placed in the bag unconscious, or he was dead before he was in the bag."
Screengrab from a video reconstruction showing a person trying to close the zip and padlock themselves in a sports holdall unaided
He suggested it would have been "very easy" to fold the dead spy's arms and place him in the holdall as long as rigor mortis had not set in.
The inquest was shown a video of Mr Faulding trying to squeeze himself into the bag while it was in a bath of the same size as the one in Mr Williams's flat.
He flayed around, starting with his torso, then tucking his head in, and finally pulling his legs in one by one.
He said: "The only way I could get myself into the bag was to lie on my back, put my shoulders and head in first, and bending my body at my stomach, pulling my knees up and pulling the bag over my body."
Mr Faulding told the hearing it would have been "extremely hot" in the holdall and Mr Williams would only have been able to survive for a maximum of 30 minutes once he was inside.
"I am used to confined spaces, and once I'm in that bag, it is a very unpleasant place to be," he said.
A second expert refused to rule out the possibility that Mr Williams locked himself in the holdall unaided.
William MacKay and a yoga-practising assistant made more than 100 attempts to recreate the feat without success.
Screengrab from a video reconstruction showing a yoga specialist trying to lock himself in a sports holdall unaided
But he said it was possible that Mr Williams, a fitness-loving maths prodigy, died without a third party being present.
"I would not like to say that it could not be done," he told the inquest at Westminster Coroner's Court.
"There are people around who can do amazing things and Mr Williams may well have been one of those persons."
Mr MacKay, who has previously worked with the military, suggested the spy would have needed extensive training to have pulled off the act in the middle of summer and in pitch darkness.
"I think you could continue to work on this for a long period of time," he said.
Video reconstructions played to the hearing showed Mr MacKay's assistant, of similar height and build to Mr Williams, curling his body inside the bag but then struggling to pull the zip shut.
Mr Williams's mountaineering experience would have given him an advantage as it would have strengthened his fingers, the inquest heard.
But Mr MacKay said: "It was very painful to do it. You tend to move the zip with your finger nails, straggling about... It was very frustrating, fiddly, you just can't get the thing together."
The expert said it was possible for someone locked inside the bag who wanted to get out to poke a small hole through the zip with their fingers.
This would give them more oxygen and allow them to unlock the padlock as long as they had the key.
"There was an option of saving oneself," he told the inquest.
Mr MacKay said the holdall was not airtight, and as long as the person inside did not panic they would be able to breathe for some time.
"If the person was in the bag willingly, then I am tending more towards the heat side being a problem than the oxygen side," he said.
"It depends on how calm you are in terms of oxygen intake."
The inquest heard that one way of getting into the holdall would be to use the "baggage handler's technique" employed by thieves to steal from locked luggage.
This involves breaking the zip apart with a pen, opening it as wide as possible, and then crawling inside and closing it again.
But in tests the zip did not close neatly, leaving a gap between the two ends, and became damaged.
Mr MacKay said it was "probably unlikely" that Mr Williams would have been able to do this successfully on his first attempt.
"There are so many intricate things to do. If he hadn't been trained, he wouldn't have had lots of knowledge, it probably wouldn't have happened," he said.
The inquest was adjourned until Monday.