A British spy found dead at his home in 2010 -- his naked body padlocked inside a large red carrying bag stowed in the bathtub -- was either suffocated or poisoned, but it is unlikely his death will ever be satisfactorily explained, coroner Fiona Wilcox said Wednesday.

Gareth Williams' death was "unnatural and likely to have been criminally mediated," she ruled.

He probably entered the bag alive, Wilcox said, reading her ruling to a court around the corner from the home of the world's most famous fictional detective, Sherlock Holmes.

But she was sure an unknown person put the bag containing his body into the bath and probably locked it, she said.

The case has gripped the British public for more than a year and a half, since Williams, an MI6 agent known for his mathematical genius and codebreaking talent, was found dead at age 31 in August 2010.

But after a weeklong inquest at which evidence from hundreds of expert witnesses, security camera images and police interviews was presented, the "most fundamental questions as to how Gareth died remain unanswered," Wilcox said Wednesday.

There was "endless speculation but little real evidence," she said.

And in unusual criticism of the Secret Intelligence Service, as MI6 is formally known, she said a delay in reporting his death and in providing relevant evidence added to the uncertainties about the case. Some of that evidence came to her only in the last 48 hours of the inquiry, Wilcox said.

The coroner appeared briefly overcome with emotion as she came to the end of reading her ruling, her voice faltering as she announced her findings.

The cause of death remains unknown, but experts agreed it was either from suffocation or poisoning, she said. "I am therefore satisfied on the balance of probabilities that Gareth was killed unlawfully," she concluded.

Among the theories aired by UK media were that Williams might have died at the hands of foreign intelligence agents or as a result of a kinky sexual encounter gone wrong, neither of which was supported by the evidence, the coroner said.

She also cast doubt on suggestions that he was into bondage or transvestism, fueled by the discovery of women's clothes, wigs and cosmetics in his apartment, saying she thought Williams bought them out of an interest in fashion, rather than any sexual motivation.

Expressing her sympathies to his family, Wilcox said the inquest could not bring him back, but she hoped it had at least allowed them to see the evidence out in the open.

A lawyer for Williams' family said that losing a much-loved son and brother would have been a tragedy in any circumstances but that the nature of Williams' death had made it all the worse.

"Our grief is exacerbated by the failure of his employers at MI6 to take even the most basic inquiries as to his whereabouts and welfare, which any reasonable employer would have taken," he said.

"We are also extremely disappointed at the reluctance and failure of MI6 to make available relevant information."

The family has called for the Metropolitan Police to review the role of the intelligence services in the investigation.

MI6 head John Sawers apologized "unreservedly" to Williams' family for the service's failure to act more swiftly and said measures have been put in place to make sure it doesn't happen again.

The detective leading the police investigation, Jackie Sebire, said new lines of inquiry had resulted from the inquest and would now be "actively pursued."

"His naked body was found in the most suspicious of circumstances," Sebire said. "It is highly likely that a third party was involved in Gareth's death, and I urge anyone who knew Gareth or who had contact with him to search their conscience and come forward with any information about what happened that night."

Inside the court, a picture emerged of a man who had few friends but was very close to his family, highly intelligent and physically fit thanks to his love of cycling.

A math prodigy who had completed his mathematics degree and master's by the age of 21, he was someone who relished a challenge and had received an award for his "world-class" work for the intelligence services.

Summing up the evidence, Wilcox said it was "extremely unlikely," but not impossible, that Williams had worked out a technique to get into the bag -- measuring just 32 inches by 19 inches (81 by 48 centimeters) -- and lock it from the inside.

But, she said, there was no evidence of footprints or handprints on the walls of the bathroom or the bath itself, as might have been expected if he had done that.

He was not a risk taker, she said, and that made it unlikely in her view that he would have put himself into the bag even as a personal challenge without making sure he could get out.