Memory sticks found among MI6 officer Gareth Williams's possessions were examined by the Secret Intelligence Service, but never handed over to detectives investigating his murder, it emerged on Tuesday.
The inquest into his death heard that the lead detective in the case, DCI Jackie Sebire, did not know until Tuesday that nine memory sticks and a holdall bag had been found at Williams's office in Vauxhall Cross, London.
The coroner hearing the inquest also accused a senior detective of not being completely impartial towards the SIS during the police investigation into the death of Williams.
Detective Superintendent Michael Broster, the SO15 counter-terrorism officer whose job was to liaise between Scotland Yard and the SIS, failed to tell Sebire about the memory sticks because he had been assured by SIS that the devices, found in a cabinet Williams shared with colleagues, were not relevant. He had agreed that the SIS should examine them.
The Westminster coroner, Fiona Wilcox, said: "You had plenty of resources at your disposal. This is a category A murder inquiry. You have access to a team who are capable of examining sticks? Looking back on that decision, remembering this is a murder inquiry, didn't you think that the team responsible for the inquiry should have investigated this evidence, and not the persons providing the evidence to you.
"It seems to me that this is a total non sequitur, and ? if this had been information provided to you by any other type of agency, you would have seized it and it would have been examined by your officers, particularly given the public anxiety around this death, which by then was huge."
The naked body of Williams, 31, a codes and ciphers expert on secondment from GCHQ in Cheltenham to MI6 in London, was found in a padlocked bag in the bath at his flat in Pimlico, central London, on 23 August 2010.
Williams' relatives gasped as Broster maintained he was "comfortable with his decision".
Wilcox replied: "It suggests you are not impartial in this case. An impartial investigator would have taken these exhibits and examined them themselves."
Broster maintained that he was "completely impartial". Questioned by the family's lawyer, Anthony O'Toole, Broster said: "We are talking about the British security services. It's not like the Met Police can just walk into Vauxhall Cross," he said.
"I believe them to be witnesses of truth. They were very helpful throughout and continue to be."
He added: "I have seen no information or evidence that someone is involved. I am not saying a member of SIS is not involved. I don't know."
Earlier, the inquest heard that Detective Constable Colin Hall was the forensic officer responsible for seizing items from Williams's office at MI6. He had not produced an inventory of either items found in Williams's shared locker, or in his locked sports bag, "due to the sensitive nature" of some of the documents. O'Toole asked: "If this had not involved the SIS, but it was the Kray twins or someone else you were investigating, you would have gone into this in far more detail, wouldn't you?"
Sebire, the lead officer in the case, said she had no knowledge of the memory sticks or the bag. "I would have expected to have been told," she said.
As the last evidence was heard, Sebire, the lead detective, said it remained her belief that "a third party" was involved in the death, and she appealed for people who may know something to "search their consciences".
The hearing continues.