MI6 and a senior detective have been accused of failing to disclose vital evidence in the death riddle of spy Gareth Williams.
A coroner suggested the counter-terror officer was not being "completely impartial" towards secret services during the Scotland Yard inquiry.
Detective Superintendent Michael Broster was criticised after his assistant was told he had offered evidence as "helpful as a London pea souper" at the inquest into Mr Williams's death.
Coroner Fiona Wilcox and the family's lawyer both delivered angry outbursts after it emerged that nine computer memory sticks and a black bag were overlooked for 21 months after the death.
The lead detective on the case was told about the evidence only on Monday.
Dr Wilcox told Mr Broster, who was unable to rule out secret service involvement in the death, that he was offering "total non-sequitur" reasons for failing to pass on evidence.
"I suggest that this means you have not been completely impartial in this case," she told him at Westminster Coroner's Court.
As Dr Wilcox ordered police to bring the missed evidence into the inquest into Mr Williams's 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 (Secret Intelligence Service) and it was the Kray twins you were investigating, you would have gone into this in far more detail."
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.