The 31-year-old codes and ciphers expert would have been overcome very rapidly by CO2, losing consciousness within two to three minutes, if the bag was sealed while he was alive.
Cause of death could not be ascertained because of the level of decomposition by the time his body was found, which meant certain poisons or volatile agents would not be detected, Dr Benjamin Swift, a consultant forensic pathologist, told the inquest. The "foremost contenders" were asphyxiation by suffocation or poisoning, he said.
The naked body of Williams was found in a padlocked holdall in his bath at his top-floor flat in Pimlico, central London, on 23 August 2010.
The Home Office pathologist Richard Shepherd said it was "more likely than not" that Williams was alive when he entered the bag, but it was "an extremely difficult call". Because of the "floppiness" of a newly dead body, it was not easy to place a body as neatly as Williams was found, he said.
Postmortem examinations revealed no evidence of "restraint, gripping or holding" marks inflicted before death. There was no evidence of strangulation or physical trauma. Williams could have entered the bag voluntarily or been coerced into it under threat, Shepherd agreed.
The independent pathologist Ian Calder, who carried out a second postmortem, said hypercapnia – carbon dioxide poisoning – would have caused a soporific effect with "gradual fading away with no pain and an increasing lack of awareness".
Asked by the coroner, Fiona Wilcox, how long that might have taken, Calder said: "Two to three minutes at best for the level to become toxic."
Small abrasions on the tips of Williams's elbows were discovered. The hearing was told these could be consistent with him moving his arms inside the bag, possibly in an attempt to escape. "Once the lock is placed on it there was no possibility of getting out of that bag," said Shepherd. "The question is: did he place the lock or did another?"
Williams would have had "a period of awareness" of being overcome by CO2, and the grazes could represent an attempt to get out, the inquest heard. "Death will come so fast, leaving little or no chance even in fit young people for there to be a response," said Shepherd. Hypercapnia was a "sign-free death", he added.
The inquest heard that the postmortem examinations were hampered by decomposition caused by heat. Despite it being summer, the radiators in the flat were on. The degree of decomposition suggested Williams had been dead for 10 days at the time of postmortem, consistent with the last time he was seen alive on CCTV on 15 August 2010.
Two hairs were recovered from the back of Williams's right thumb and his right little finger. The coroner said no forensic analysis of them had been submitted to the inquest, and she asked for any details to be disclosed.
DNA from another person was found on Williams's left hand. The inquest was told that details of the DNA were erroneously inputted into a database by LGC, the forensic company analysing samples, an error only discovered a year after Williams's death. The DNA sample was from one of the forensic scientists at the scene. Paul Allen, of LGC, expressed his "very deep regret" to Williams's family for the error.