The body of a suspected British spy could have been left undiscovered for up to a fortnight.
Detectives are now investigating the death of a man in his 30s, after he was found in a top-floor flat in Pimlico, central London.
Officers came across a mobile phone and several SIM cards laid out nearby when they broke into the top-floor flat in Alderney Street.
It is thought the corpse was stuffed into a bag and left in the bathroom.
It is understood the man was employed as a communications officer at the GCHQ "listening post" in Cheltenham, Gloucestershire.
But he is believed to have been on secondment to the riverside headquarters at MI6, the Secret Intelligence Service, about half a mile from the flat.
Police continue to scour the two-floor flat for evidence and cordons remained in place on the prestigious street where two former home secretaries live.
Ex-Tory leader Michael Howard and Sir Leon Brittan are among a host of politicians and bankers who have homes there, residents said.
Neighbour Laura Houghton said an "extremely friendly" man identifying himself as Gareth lived at the address at the centre of the murder inquiry.
The 30-year-old secretary said: "I have spoken to him only once. I met him in the entrance hall of the set of flats because of a boring plant issue about a year ago.
"He was extremely friendly and had a Welsh accent."
Mrs Houghton added: "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."
Eileen Booth, 73, who lives opposite, said detectives told her the murder may 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."
Scotland Yard has launched a murder investigation, led by detectives from its Homicide and Serious Crime Command wing.
A spokesman said the body was yet to be formally identified and a post-mortem examination would be taking place.
