MI6 has been accused of failing to disclose evidence during the 21-month police inquiry into the death riddle of spy Gareth Williams.
A detective faced claims he offered information as "helpful as London pea souper" after it emerged secret services held on to nine computer memory sticks and a black bag.
As a coroner ordered police to bring the missed evidence into the inquest of Mr Williams' death, family barrister Anthony O'Toole told police they had not taken the incident seriously enough.
The lawyer said: "If this had not involved SIS and it was the Kray twins you were investigating, you would have gone into this in far more detail."
The North Face bag - similar to the one in which Mr Williams was found dead - was discovered by officers under the spy's desk at MI6's HQ. But the lead detective on the case was only told about it on Monday after officers were stopped from seizing it in August 2010. MI6 also examined computer equipment belonging to Mr Williams without telling police, the inquest heard.
Family members of Mr Williams shook their heads as Detective Constable Colin Hall, of the force's counter-terror SO15 branch, said his search of the agent's Vauxhall HQ was called off shortly after the spy was found dead.
Mr Hall said "there was stuff in there of a sensitive nature" in the bag but, when asked what, he said he could not remember. Mr O'Toole added: "That's about as helpful as London pea souper."
Detective Chief Inspector Jackie Sebire, the lead officer on the case, said she had no knowledge of the existence of memory sticks in Williams cabinet at work. She told Westminster Coroner's Court: "I would have expected to have been told."
Fitness enthusiast Mr Williams, 31, was found naked, curled up in a padlocked holdall in the bath of his flat in Pimlico, central London, on August 23 2010.
Pathologists said on Monday that he would have suffocated within three minutes of getting inside the bag. Poisoning and asphyxiation are the "foremost contenders" in solving the death riddle, they added.