MI6 codebreaker Gareth Williams was probably killed but the "spy in the bag" case might never be solved after mistakes by investigators, an inquest heard.
Mr Williams's relatives attacked failures by secret services & police after a coroner ruled "many agencies fell short" in their investigation.
Fiona Wilcox said she was sure a third party locked the 31-year-old mathematics prodigy inside the red holdall, probably while he was still alive.
She criticised the 21-month investigation, saying it was unlikely the mystery "will ever be satisfactorily explained".
"The cause of his death was unnatural and likely to have been criminally mediated," she said during a two-hour narrative verdict at Mr Williams's inquest.
"I am therefore satisfied that on the balance of probabilities that Gareth was killed unlawfully."
The Metropolitan Police's Deputy Assistant Commissioner Martin Hewitt said they are trying to "trace unidentified DNA" to understand what happened to Gareth Williams.
He added that the case "remains a current investigation".
The family of MI6 spy Gareth Williams have hit out at the failures of MI6 to raise the alarm after the spy went missing, saying their "grief is exacerbated" by it.
In a statement read out by their solicitor after the inquest into his death concluded, they said they were "extremely disappointed" at the secret services' "reluctance and failure" to make relevant information available to the death inquiry.
They also attacked the "total inadequacies" of the inquiry by Metropolitan Police counter-terror branch SO15 into MI6 and called on Scotland Yard's chief to look into how the investigation would proceed in light of this.
One of Britain's most senior spies apologised to Gareth Williams' family today for the mix-up which saw MI6 fail to report his disappearance for a week.
Sir John Sawers, chief of the Secret Intelligence Service, expressed the "deepest condolences" of MI6 and GCHQ for the death.
In a statement delivered by MI6 lawyer Andrew O'Connor, Sir John said the service should have acted more swiftly when Mr Williams failed to turn up to work in August 2010.
Mr O'Connor said: "On behalf of the whole organisation, Sir John regrets this deeply and apologises unreservedly."
On balance of probabilities, coroner agrees with family that Gareth was unlawfully killed - though that's not the actual verdict #spook
Coroner thanks DCI Jackie Sebire who is leading police inquiry and gives her sympathies to Gareth's family #spook
MI6 barrister on his feet says chief John Sawers recognises "anguish" delay in finding Gareth causes his family an apologies "unreservedly"
Coroner says "beyond belief" Gareth could have arranged the lock and zips so neatly from the inside #spook
Coroner says on balance of prob Gareth entered the bag alive and died from hypercapnia or unknown poison #spook
Coroner says women's clothes too small for Gareth to wear, could have been presents, lack of underwear 'inconsistent' with transvestitism
No evidence to support idea Gareth was a transvestite or gay - friends evidence goes against that completely #spook
The coroner ruled out Mr Williams's interest in bondage and drag queens in having any bearing on his death.
Instead, she questioned leaks about his private life to the press.
She said: "I wonder if this was an attempt by some third party to manipulate the evidence."
The coroner suggests that inaccurate reporting of Gareth Williams' personal life was an attempt by a third party to manipulate perceptions of the case.