It had all the hallmarks of a classic locked room mystery. The body of a British spy found dead inside a sports bag, its zipper padlocked shut from the outside.

Incredibly London’s metropolitan police concluded the death was “probably an accident”. Their conclusion overturned an official inquest’s findings that the death was “unnatural and likely to have been criminally mediated”.

Unusual features of the crime scene made the police’s job virtually impossible from the start. Even though it was the middle of summer, the heating in the flat had been left on at the highest setting, hastening the decomposition of Williams body and making it impossible for the pathologist to ascertain the cause of death.

Odder still was the lack of DNA or fingerprints in the bathroom and on the bag. Even if William’s could have somehow locked himself in the holdall, how could he have done so without leaving any forensic traces behind?