Police investigating the death of a spy whose body was found in a holdall are in direct contact with the head of MI6, a senior officer has said.
There is a "very good line of communication" with the intelligence service over Gareth Williams' mysterious death but the investigation remains "tricky", assistant commissioner Mark Rowley said.
The codebreaker was found naked in the bag which was in the bath at his flat in central London in August 2010, and no significant progress has been made in finding out how he died.
Earlier this year Met commissioner Bernard Hogan-Howe ordered that detectives investigating his death must have direct contact with MI6.
Previously investigators were forced to involve counter-terror colleagues in an attempt to obtain statements and evidence but Hogan-Howe was angered by delays in passing information to a senior investigator.
Rowley said: "We've got access to everyone we need to speak to. We've got a much clearer arrangement and got a direct line of sight and communications. I can speak personally 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."
In May coroner Fiona Wilcox said that 31-year-old Williams was "probably" killed and that she was sure someone else locked him in the bag. But she said it is "unlikely" that the mystery would ever be solved because of mistakes by investigators.
Dr Wilcox said several factors hampered inquiries, such as breakdowns in communication by her own coroner's office, a DNA mix-up by forensics and the late submission of evidence by MI6 to police.
She also questioned why details of Mr Williams's private life were leaked to the press. It was suggested he had an interest in bondage and drag queens.
Rowley said the fitness enthusiast, originally from Anglesey, North Wales, was difficult to understand.
"Williams is a challenging guy to understand, his personal life and his circumstances, his history. We've got to try and understand what was going on and what led to such an unusual and suspicious death. But we've got full co-operation," he said.
"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 was present."
Related on HuffPost: