'SPY in a bag' Gareth Williams did lock himself inside a holdall, police are now said to believe.
Scotland Yard detectives reportedly think the GCHQ codebreaker must have climbed inside the sports bag where his naked body was found and have ruled out an assassination.
Mr Williams was on secondment to MI6 when his body was found curled up inside a padlocked red North Face holdall in the empty bathtub of his apartment in Pimlico, central London, in August 2010.
The keys to the padlock were also found inside the bag.
An inquest in May concluded the 31-year-old was most likely poisoned or suffocated.
Coroner Dr Fiona Wilcox said she was sure that someone else had locked Mr Williams in the holdall after two experts tried 400 times to lock themselves inside the bag.
One claimed even Harry Houdini "would have struggled" to squeeze himself into the holdall.
Days after the inquest verdict a retired army sergeant showed how it was possible to climb into a similar bag and lock it from inside.
Following the inquest ruling, Scotland Yard's murder squad vowed to investigate new evidence.
A report by The Daily Telegraph says detectives have now concluded he probably died alone and are preparing to present their new findings to the coroner.
It also says they believe the 'only explanation left' is that Mr Williams died as a result of 'autoerotic activity'.
A spokeswoman for Scotland Yard said: "This remains an active investigation and officers continue to explore lines of enquiry.
"Officers remain open-minded in relation to the circumstances of the death of Mr Williams."