A British spy found murdered in his flat might have lain undiscovered for up to two weeks.
The man, named locally as Gareth Williams, was found stuffed in a large sports holdall in the bath of his central London home.
Mr Williams, aged in his 30s, was employed as a communications officer at the GCHQ "listening post" in Cheltenham, Gloucestershire.
He was on secondment to the riverside headquarters of MI6, the Secret Intelligence Service, about half a mile from the flat.
Officers discovered Mr Williams after breaking into the flat on Monday afternoon when the alarm was raised by colleagues who had not seen him for "some time".
They found his decomposing body, as well as his mobile phone and several Sim cards, laid out nearby at the top-floor flat in Alderney Street, Pimlico.
A post-mortem examination was being carried out by a Home Office pathologist today to establish what led to his death.
Sources close to the inquiry said it is not clear how he died and played down speculation that the murder is linked to his secretive line of work.
One source said: "The suggestion there is terrorism or national security links to this case is pretty low down the list of probabilities."
Neighbours described Mr Williams as an "extremely friendly" and athletic man who enjoyed cycling and had a strong Welsh accent.
They said he had lived in London for about a year and was planning to return to Cheltenham, where he rented a flat.
A postwoman who called at the block today said he often collected parcels at the communal front door.
Secretary Laura Houghton, 30, said: "His windows were always shut and curtains were often closed. I could never tell if anyone was in.
"It was strange that we never saw him come and go. I just assumed he worked away.
"The first I heard of anything happening was when the police knocked on my door and asked me if I had heard anything happening. I told them the walls were so thick that I couldn't hear a thing.
"All they told me was that there had been a serious incident. I'm amazed it's taken this long to all come out."
Eileen Booth, 73, who lives opposite, said detectives told her the murder might have taken place two weeks ago.
She said: "A few years ago, I would definitely have known who it was that had been killed. But nobody knows each other these days.
"Detectives came round and asked for our eye colour and height. They said this probably happened two weeks ago."
The scene of the murder is a two-storey flat on a prestigious street among a row of expensive five-storey Victorian townhouses.
Residents are mainly bankers and politicians, including former home secretaries Michael Howard and Lord Brittan.
The ownership of the building is hidden behind the private company New Rodina, registered in the British Virgin Islands.
The property was bought for £675,250 in 2000, remortgaged twice and the word rodina means "motherland" in Russian and Bulgarian.
Public documents revealed several current and former residents of the freehold block have links to London and Cheltenham.
One Frenchman who lived at the flat between 2005 and 2006 is an expert in global satellite positioning, radio communications and high sensitivity antennae.
Officials at Scotland Yard's Counter Terrorism Command and domestic intelligence agency MI5 have been kept up to date about the police inquiry.
The murder inquiry is being led by officers from Scotland Yard's Homicide and Serious Crime Command.
A spokesman said the body was yet to be formally identified and a post-mortem examination would take place later today.
He said investigators were following up "several lines of inquiry" but declined to confirm the occupation of the dead man.
It is the first murder on British soil of someone linked to the secret services since the death of Alexander Litvinenko in 2006.
The former KGB agent died in hospital after drinking tea laced with radioactive polonium-210.
Bulgarian defector Georgi Markov was killed by an assassin who used an umbrella to fire a deadly ricin pellet into his leg as he walked across Waterloo Bridge in September 1978.
A Foreign Office spokesman said: "There is an ongoing police investigation.
"It is long-standing Government policy not to confirm or deny that any individual works for the intelligence agencies."
A GCHQ spokesman said: "There is an ongoing police investigation and it would not be appropriate for us to comment at all as this is ongoing.
"We have nothing to add. Our policy is not to comment on individual members of staff or whether they are staff."
Mr Williams, 31, joined the University of Cambridge in 2000 to undertake a postgraduate certificate in advanced studies in mathematics but dropped out.
A spokeswoman said he was a member of St Catharine's College but left without completing the qualification the next year.
The course is described as "demanding" and normally only accepts students with first-class degrees in physics, mathematics or engineering.
Forensic officers continued to comb the scene this afternoon.
Three officers in pale blue overalls were seen returning to the townhouse shortly after 3pm.
William Hughes, a relative of Mr Williams, said he had no idea of the type of work the spy was engaged in at GCHQ.
He told the BBC: "He worked for GCHQ for many years. I knew he was working in London doing something.
"He would never talk about his work and it felt rude to ask really."
Mr Hughes said he had not yet spoken to the dead man's parents.
He added: "Unfortunately they were away out of the country, they have arrived back in the country today, I haven't spoken to them yet.
"It was terrible when I had the phone call yesterday morning, I just didn't know what to believe really, I didn't know what to believe."
He said Williams was "very, very talented", adding: "We don't know what he was doing, we never spoke about it."