A pistol used in the murder of Belfast solicitor Pat Finucane was handed back to the British Army by the RUC, previously unpublished papers show.
New details about collusion between the RUC and the loyalist killers who targeted the 38-year-old in 1989 have been revealed in a report.
The unseen chapter from the Stevens Inquiry is highly critical of the RUC's "inadequate" investigation in the case and found officers deliberately destroyed vital evidence, while exhibits and records could not be found and fingerprints at the scene were not compared against suspects.
It stated one of two murder weapons, a Browning pistol, was recovered by police but then given back to the British Army, from where it had previously been stolen by loyalists.
Mr Finucane's son Michael said he is not surprised by the revelations.
"Unfortunately, many other families are in a similar position to ourselves where they are finding out after the fact because the material has been held back for so long, that what they were told was a diligent and efficient and effective investigation was in fact anything but," he said.
Four chapters of a report by Sir John Stevens were published in 2003. In the newly-released nine-page chapter six, entitled Murder Investigation, Mr Stevens criticised the handling of the murder weapon by the RUC.
"This was not a case of administrative oversight, or even some loss occasioned by a lack of care," he wrote. "I believe it was a clear and deliberate decision to relinquish control of a key exhibit, resulting in the destruction of vital evidence. The lack of records has prevented the identification of the person responsible for this decision.
"The potential consequences of this particular disposal are obvious, with allegations made from the start of collusion between the security forces and loyalist paramilitaries. The face that the firearm, when recovered, was found to have originated from the army no doubt fuelled that suspicion."
Chapter six was released following a four-year battle by RTE reporter Richard Dowling under the Freedom of Information Act in the UK. Catholic father-of-three Mr Finucane was shot dead when loyalist UDA/UFF gunmen used sledgehammers to burst in through the front door of his home in north Belfast in February 1989.