MI6 is investigating why it took two weeks to discover that a GCHQ officer on secondment to the agency had been murdered.
Gareth Williams's body was found stuffed into a sports bag in the bath at his home in central London by Metropolitan Police officers. They forced their way into the flat on Monday afternoon after colleagues reported he had not been seen for some time.
While police and MI6 officers do not believe the 31-year-old's murder had anything to do with his work as a codes and ciphers expert, they are concerned that any agent could go missing for such a long time without checks being made on their whereabouts.
Investigators suspect Mr Williams may have known his killer as there was no sign of forced entry at his top-floor flat in a smart street in Pimlico where his neighbours included wealthy bankers and politicians, including former home secretaries Michael Howard and Lord Brittan.
Scotland Yard's Homicide and Serious Crime Command was investigating whether the victim's private life or relationships offered any clues that could identify his murderer. As a fingertip search was conducted on the property, police have been examining his telephone and financial records, as well as CCTV footage from the area. A post-mortem examination proved inconclusive and detectives are now awaiting the result of toxicology tests.
Before gaining his post at the national "listening post" Government Communications Headquarters (GCHQ), Mr Williams would have undergone strict vetting including a nine-month process of interviews, background checks and a medical. As well as examining his family history and personal circumstances, it would assess whether he had the character for the secret world of intelligence work.
However, there were questions raised last night as to whether he may have held something back from his employers. Family and friends painted a picture of an athletic young man, a keen cyclist and runner, who was deeply private and rarely socialised.
His mother's cousin, William Hughes, said: "I knew he worked at GCHQ and he had been working in London but I didn't know what he did. It wasn't said that we shouldn't talk about it, I simply never asked and he never told me. He was a bright boy from a young age and his parents were very proud of him."
Mr Hughes added that his parents, Ian and Ellen, as well as his younger sister, Ceri, were deeply shocked and devastated by the news. His mother and father, from Wales, were picked up by police upon their return from a holiday in America on Tuesday and taken to identify his body in London.
A childhood friend, Dylan Parry, 34, described Mr Williams as academically gifted but socially naive, an isolated boy fascinated by mathematics and computers. Mr Parry, who went to school with Mr Williams at Uwchradd Bodedern secondary in Anglesey, said that he travelled to Bangor University every week aged 16 to study for a mathematics degree part-time. He later graduated from the university with a first class degree.
"It was clear he was going to go far, but we all assumed he would end up in academia. Finding out he became a spy was a shock," he added.
Keith Thompson, of Holyhead Cycling Club, said he had known Mr Williams since he joined the club at the age of 17. He said: "It's true that he was very quiet. He wasn't a great conversationalist. We were club mates but Gareth wasn't the sort to go to the pub after a race so he didn't have any close friends in the group. I never spoke to him about his job or his private life. Nobody did with Gareth."
Charles Cumming, author of A Spy by Nature, who was approached himself for recruitment by the SIS after university, said the secretive nature of the work and the long hours could prove a real burden on relationships and many chose to mix within the services.
But he added: "I think the pressure that spies are put under has been exaggerated by programmes like Spooks. People who go into the work are highly intelligent, highly motivated and well aware of the sacrifices they are making. To a man or woman they don't find the restrictions all that difficult."
While MI6 remained an "alpha male" culture, he added, GCHQ was a very different environment.