The gun used to shoot Garda Anthony Golden in October 2015 was the same model that his killer, Crevan Mackin, had admitted to importing nine months before the murder, writes Paul Murphy.
Mackin, who shot his partner Siobhán Phillips in the head in the same incident in Omeath in Co Louth, before killing himself, made the admission when he was arrested by detectives in January 2015 following an FBI tip-off to gardaí that he had bought bomb making material and weapons parts online and imported them to Ireland.
A Garda ballistics report seen by RTÉ Investigates reveals that the gun used to kill Garda Golden on 11 October 2015, was a Glock 9mm pistol.
Mackin also admitted at the time to importing a Glock 22 .40 calibre Smith and Wesson pistol.
After the murder of Garda Golden, the same model of Glock 22 was found in Mackin's car along with over 300 rounds of ammunition.
While Mackin admitted to importing parts to make six guns, he only provided information on the whereabouts of two of them.
Despite this, he was released on bail.
Mackin, who had links to dissident republicans, was charged at the time with IRA membership, a charge he had consistently denied during Garda interrogation.
He was not charged with weapons or bomb making offences, both of which he had admitted during questioning.
Ms Phillips' father, Seán Phillips, described the decision to grant Mackin bail as "ludicrous, it's not right, it's unforgivable."
The Garda ballistics report described the gun that Mackin used to kill Garda Golden as made from parts that were "…sourced separately and subsequently married together to form a single functioning firearm."
Mackin, who was unemployed, had admitted to his sister after his January 2015 arrest that he made extra money from importing weapons parts and decommissioned guns and turning them into viable weapons for sale to dissident republicans.
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Murder the subject of three GSOC Investigations
The murder of Garda Golden and the maiming of Siobhán Phillips are the subject of three separate investigations by the Garda Síochána Ombudsman Commission.
Louth TD Gerry Adams, who has previously criticised how Mackin was dealt with by the authorities, has also questioned the pace of official investigations. He says that he sent the Garda evidence file to the then justice minister Frances Fitzgerald in late 2015, soon after the murder.
He said that two-and-a-half years later "there is still no adequate effort to make this case transparent and those involved accountable."
GSOC announced that it had begun a public-interest investigation 12 months ago but in a letter to Mr Phillips' solicitors dated 27 March 2018, GSOC Senior Investigation Officer Jon Leeman wrote: "I am sorry we have not made better progress.
"The resourcing of the investigation and our office in general, is causing great difficulties and hampering the progress of the investigation".
The letter also stated that unspecified information sought from An Garda Síochána had not yet been provided to GSOC.
Three months before that letter, the Garda told the Department of Justice that "...all materials sought by GSOC have been fully complied with, save for records held at Security and Intelligence section, which are being dealt with under the relevant protocols governing sensitive material."
Fianna Fáil Justice spokesperson Jim O’Callaghan described the failure to supply GSOC with the information it sought as "a matter of concern."
"GSOC should receive full information from An Garda Síochána if it requests that information".
Mackin's sister said she believes her brother received lenient treatment by the State in return for agreeing to work for gardaí as an informer. However, given his brittle and volatile mental state she questioned if this was an appropriate role for him.
"Police forces all over the world need and operate with informers," she said. But she added that gardaí would have been aware that her brother "needed medication and it was for mental health concerns and disorders".
RTÉ Investigates has seen files detailing Mackin's medical history.
He was on anti-anxiety medication at the age of nine and later self-harmed. He was diagnosed with Aspergers syndrome and was treated for ADHD, depression and anxiety.
He was arrested after he threatened to kill his mother when he was 16.
He spent time in a psychiatric hospital two years later.
Thirty months on from the murder, Mr Phillips said he is no nearer to getting an adequate explanation regarding Mackin's interactions with An Garda Síochána prior to the day of the murder.
He is calling for a public inquiry into issues surrounding the events of 11 October 2015.
"We need a public inquiry so this will never ever happen again," he said.