(CNN) -- The death of MI6 codebreaker Gareth Williams -- whose naked body was found inside an externally locked bag in his bathtub in 2010 -- was a "perfect crime," a confined spaces expert says.
Peter Faulding said he disagreed with Scotland Yard's conclusion that Williams most likely locked himself in the bag, saying it was his belief that the MI6 worker was murdered.
Last week, London's Metropolitan Police said its three-year investigation had found a lack of evidence to show that someone else had been involved in Williams' death. The police position differs from a 2012 coroner's report, which said it was likely he had been killed.
Faulding testified at the Coroner's inquest and did not definitively rule out that Williams could have somehow locked himself into the bag alone. But he said he could not have done so without leaving evidence.
This week Faulding told CNN he was still of that view, using the same model of bag and a similar type of bathtub to show how Williams could have gotten into the bag and why he held the view that someone else was involved.
"There was no DNA found on the padlock, the zipper closures -- anywhere around this bath," he said. "To do this without leaving a trace is absolutely impossible. Even Houdini couldn't do this."
Faulding said the lack of DNA was "the key to this crime."
"We can zip ourselves in the bag ... but you can't do it without leaving a trace."
Faulding said a scuff mark was found on the bath, and it was his belief that Williams was murdered.
"The bag was lifted. It scuffed the top and he was dropped into the bath," he said.
With the heating in Williams' apartment turned up, Faulding said "the decomposition fluids would actually run down the plughole, and it would stop the smell of decomposition escaping the flat."
He concluded: "This was the perfect crime."
The Met last week acknowledged that some of the evidence -- including the lack of DNA on the padlock or hand prints on the bath -- was "odd."
It said there was insufficient evidence to be definitive on how Williams died but that police believed that theirs was the "more probable" conclusion.
The case gripped the British public when Williams was found dead at age 31 in August 2010.
Among the theories aired by UK media were that Williams might have died at the hands of foreign intelligence agents or as a result of a kinky sexual encounter gone wrong.
While the circumstances of Williams' death remain somewhat murky, there is still forensic evidence left to examine. More than 10 samples of DNA found in Williams' apartment were too small to test.
Police say that maybe, one day, technology may help solve the mystery.