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."
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.