A coroner said Wednesday a British spy whose naked body was found inside a locked sports bag was likely killed in a criminal act, but acknowledged the riddle of how he met his mysterious death may never be solved.

Coroner Fiona Wilcox said it was unlikely that the demise of code breaker Gareth Williams, 31, would "ever be satisfactorily explained," despite a 21-month police inquiry, and seven days of expert evidence to an inquest hearing.

Williams worked for Britain's secret eavesdropping service GCHQ but was attached to the MI6 overseas spy agency when his remains were found in August 2010 at his London apartment, in the bag and inside a bathtub.

Wilcox said the spy was likely killed either by suffocation or poisoning in a "criminally meditated act" and acknowledged it was possible that an intelligence agency colleague may have been involved.

It is "likely that Gareth entered the bag alive and then died very soon afterwards," the coroner said as Williams' family listened to her verdict inside Westminster Coroner's Court.

The coroner moved to dispel wild theories that have swirled around the case -- insisting there was no evidence of a sexual encounter gone wrong, of suicidal intent, or that Williams' death was linked to a supposed interest in bondage.

She said there was no evidence that the death was tied to work, amid speculation the spy could have been the target of Russian criminal gangs or an al-Qaida extremist.

Wilcox said it appeared "extremely unlikely" that Williams could have climbed inside the sports bag and locked it himself. Two different specialists attempted to recreate the feat hundreds of times without success.

Given testimony that described Williams as deeply private -- shunning his colleagues outside of work -- and wary of strangers as a result of his secret work, she said a potential killer must have been a friend, or entered his home uninvited.

Williams was discovered in the fetal position inside the bag, his arms calmly folded across his chest and with two keys to the bag's padlock underneath his buttocks.

Pathologists told the inquest that poisoning or asphyxiation may have killed Williams, but acknowledged they can't be certain because his cadaver badly decomposed as it lay undiscovered for several days.

Wilcox said that while there wasn't evidence to support a specific verdict of unlawful killing -- which would need a high burden of proof -- it was her opinion that the spy was probably unlawfully killed.

She said that while it appeared unlikely, it continued to be a "legitimate line of inquiry" that British intelligence agencies may have had a role.

MI6 colleagues failed to report Williams as missing for a week, meaning that police and medical experts lost vital chances to gather evidence. Detective Chief Inspector Jackie Sebire also told the inquest that British spy services had failed to pass on evidence in the case until this week: nine computer memory sticks that had been found at Williams' workplace.

John Sawers, the head of MI6, said in a statement released following the verdict that he apologized "unreservedly" to the Williams family for the spy agency's failure.

Police are uncertain exactly how Williams died and have so far made no arrests, though their investigation into the spy's death remains active.

Family lawyer Anthony O'Toole has previously said Williams' relatives suspect "the dark arts of the secret services" may have been behind a cover up of the true circumstances of his death.

Wilcox dismissed claims that a fetish for sadomasochism or transvestitism may have been behind Williams' death. There was no evidence the spy was interested in any such thing, Wilcox concluded. She added that she believed rumors may have been stoked by a "third party to manipulate the evidence," without elaborating.

Wilcox delivered a so-called "narrative verdict" -- a lengthy but inconclusive ruling which is available when there isn't clear evidence of natural causes, suicide or enough proof of unlawful killing.

In Britain, a corner's task is to outline the circumstances of how and when a person died -- rather than apportion blame, or recommend criminal charges.