MI6 was accused today 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 which Mr Williams was found dead in at his flat - 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 yesterday 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 his cabinet at work.
She told Westminster Coroner's Court: "I would have expected to have been told."
She added: "What I knew was that Gareth's email accounts had been checked but I did not know that other media had been checked."
Mr Hall was ordered to re-examine the black bag in the courtroom as the inquest heard its last day's evidence.
The 31-year-old fitness enthusiast was found naked, curled up in a padlocked holdall in the bath of his flat in Pimlico, central London, on August 23 2010.
Coroner Fiona Wilcox asked Mr Hall: "Would you not consider that seizing another sports bag might be relevant?"
He replied: "I was told there was nothing there about Gareth's death."
He later said: "I will do what I'm told."
He added that "we had not completed our search" when he was told to wrap up efforts on his first visit on August 26.
Pathologists said yesterday that Mr Williams would have suffocated within three minutes after getting inside the bag.
Poisoning and asphyxiation are the "foremost contenders" in solving the death riddle, they added.