As an inquest into the death of MI6 officer Gareth Williams finds "on the balance of probabilities" he was unlawfully killed, Peter Faulding, an expert on confined spaces - and who gave evidence at the inquest - says he is convinced Mr Williams was dead before he was put inside the bag.
During seven days of inquest, the question of whether the 31-year-old codebreaker could have padlocked himself in a bag in a bath was central - and the footage of Peter Faulding struggling to do just that was circulated world-wide.
Speaking to the BBC, Mr Faulding said he had tried and failed 300 times. "Even Harry Houdini wouldn't have managed it," he said.
Both Mr Faulding - who is 5ft 6in (1.7m) tall and weighs 70kg (11st) - and the dead MI6 officer were of "similar" build.
He says getting into the bag was not a problem - the critical issue is the way in which the bag had been padlocked.
"Gareth could have got into the bag: I put my head in first, followed by the shoulders and then the leg. You can get into the bag and you can zip the bag up. That's not a problem," he said.
"The problem is when it is in the bath, putting the padlock on and actually securing the bag from the inside.
"Gareth was found in a very peaceful position, so there was no struggle" he added.
Mr Faulding believes that the MI6 officer would have to have been dead before his body was put into the bag.
"You can't access the lock from the inside without damaging the bag. So it cannot be padlocked from the inside. And the lock was left in a strange position."
Absence of any traces of DNA comfirms Mr Faulding's belief that Mr Williams was dead before he was put inside the bag.
"There would have been prints. There would have been DNA left on the bath as Gareth lowered himself inside the bag. But there was no DNA, no fingerprints, nothing actually at the scene.
"The door was closed and the key to the bag was actually under Gareth's right buttock. The heating was turned on fully in August.
"My belief is that the bag was placed in the bath with Gareth dead already.
"I believe there was definitely a third party involved or third parties."