A man who shot dead three women had been allowed to keep his guns by police despite complaints of domestic violence and then threatening to shoot himself, according to a report seen by the BBC.
Taxi driver Michael Atherton, 42, killed his 47-year-old partner Susan McGoldrick, her sister Alison Turnbull, 44, and her niece Tanya Turnbull, 24, before taking his own life on New Year's Day.
The four bodies were found in a semi-detached house in Horden, near Peterlee, County Durham.
The BBC's Inside Out North East & Cumbria has seen a report by the Independent Police Complaints Commission (IPCC) on the handling of Atherton's firearms certificate.
The IPCC is not due to officially release the report until the conclusion of inquests into the four deaths.
But according to Inside Out, the report brands Durham Police's failure to remove Atherton's guns "inexcusable" and "unacceptable".
It said the report details how a note was stuck to Atherton's file when he applied for his licence which said: "4 domestics - last one 24/4/04 . . . . . would like to refuse - have we sufficient info - refuse re public safety".
But this recommendation was overruled because it was thought Atherton would win an appeal.
He was given a licence to keep six guns and warned it would be revoked if he behaved "irresponsibly".
Officers removed his guns after a drunken incident in 2008 during which he threatened to "shoot his own head off".
But they were returned six weeks later.
The programme said the IPCC has not recommended criminal or disciplinary action but found there was no "meaningful review" of Atherton's licences by police after the 2008 incident and no record of why the decision was made.
Laura McGoldrick, who escaped from the house, told Inside Out: "I know I'm not Einstein or anything (but) if a man tried to commit suicide, or pretended to, why would you give a man a gun back like that with a family in the house?"
Durham's Deputy Chief Constable Michael Banks told the programme he was aware the families of those who died had been updated by the IPCC.
He confirmed the commission had recommended that there were no criminal or misconduct cases to answer for any member of Durham Constabulary.
Mr Banks said: "At the conclusion of the inquest, Durham Constabulary will respond in full to all of the issues raised in the IPCC's report. Since the tragic events in January, we have carried out a full review of policy and procedures.
"It would be wholly inappropriate to comment further on the outcome of an investigation in advance of the official publication of the findings, and the coroner's inquest.
"We will not be making any further comment at this time."
The IPCC said it could not comment.
:: Inside Out North East & Cumbria is on BBC One tonight at 7.30pm.