MI6 co-operates with police over Gareth Williams death
Police investigating the death of the MI6 officer Gareth Williams say they are in direct contact with senior staff at the intelligence agency.
The code-breaker from Anglesey, who was 31, was found locked in a sports holdall at his central London flat in August 2010.
One Scotland Yard officer described the unsolved case as "tricky".
In May this year a coroner recorded a narrative verdict, saying it was likely that Mr Williams was killed unlawfully.
Assistant Commissioner Mark Rowley said MI6 was providing detectives with information.
"We've got access to everyone we need to speak to," he said. "I can speak... direct to the head of Six, so we've got a very good line of communications. But it remains a tricky case.
"On the one hand, of course you need to respect national security and on the other hand, of course you need to do a penetrating and thorough investigation.
"Squaring that circle is a challenge, and what we've learned is that the way we tried to square that circle in the first stage of the investigation was not quite right."
The naked body of Mr Williams was found in the bag in the bath of his home in Pimlico.
By the time police were alerted to his disappearance, he had not been at work for a week and there was extensive decomposition of his body.
At the inquest the coroner, Fiona Wilcox, said: "Most of the fundamental questions in relation to how Gareth died remain unanswered," including how the bag and body came to be in the bath.
After the verdict, the Metropolitan Police said they intended to re-focus their efforts, based on the evidence heard and the new lines of inquiry that had emerged during the hearing.
Officers said they intended to develop DNA profiles they already possessed, and analyse telephone communications, to try to shed light on Mr Williams's final days.
Assistant Commissioner Rowley said: "Williams is a challenging guy to understand, his personal life and his circumstances, his history.
"People can come to their own conclusions without knowing all the evidence about exactly how the bag was secured. We're still working on the basis that we expect there was somebody else present."