At the inquest on Friday, a video was played showing two men hired by the police — one a yoga expert and the other a former military man trained in escape techniques, and both of roughly the same size and height as the muscular, athletic, 5-foot-7-inch Mr. Williams — trying to replicate what he would have had to do to get himself into the bag alone and lock it from inside. Using the same kind of red, extra-large North Face duffel bag, and the tub in the Pimlico flat, the two men were shown contorting themselves — more than 100 times in the case of one man, and 300 times in the case of the other — without managing the feat.

Investigators concluded that someone else had to have helped in closing the bag and locking it. The police have said further that they cannot rule out that Mr. Williams was dead before being placed in the bag, or that the bag with his body in it was lifted into the bathtub from somewhere else. There were no fingerprints or other traces that would have been expected if Mr. Williams had supported himself on the bathtub’s rim while lowering himself into the bag.

Concerns about national security have contributed to the 20-month delay in the inquest.

Mr. Williams, a Cambridge-educated mathematical genius from the mountains of North Wales, was working on what his superiors have described as the practical use of new technologies in the field of electronic surveillance. Police testimony has described him as a picture of tranquillity in death, lying faceup, looking “very calm,” with no injuries to his nails or fingers and no “signs of stress or fear” on his body or on the bag’s interior netting. But the men who tried to lock themselves in the bag, and pathologists, have said at the inquest that he would have suffocated within 30 minutes from a rapid 20-degree rise in heat and a buildup of carbon dioxide.