The Independent Office for Police Conduct (IOPC) has completed its investigation into Jake Davison’s possession of a shotgun and shotgun certificate.
The 22-year-old killed his mother Maxine Davison, 51, at her home in Keyham before going out into the street and shooting dead four strangers in a 12-minute attack.
The killings happened a matter of weeks after the shotgun and licence had been returned to him by Devon and Cornwall Police.
They had been seized last year after Davison assaulted two teenagers in a park. He admitted the offences and attended a voluntary Pathfinder programme.
The Independent Office for Police Conduct (IOPC) has now completed its investigation into how the force approved his application and then later gave him back the licence and shotgun.
The IOPC says its investigation report includes its view on whether any individual whose conduct has been investigated may have a disciplinary case to answer, along with a series of recommendations around current firearms licensing procedures.
It has included issues around firearms licensing law, guidance and procedure, Jake Davison's engagement in the Pathfinder scheme, his medical records, education, employment and firearms licensing history and the weapon and ammunition he possessed.
It has been submitted to Devon and Cornwall Police and the Coroner, but details of the investigation will not be made public until the inquest has finished in January next year.
IOPC regional director David Ford said: “We fully understand the importance of providing information and answers to the many questions arising from this tragic incident.
"However, although our investigation is now complete, we are unable to publish our investigation findings at this time because we need to ensure that we do not prejudice any future proceedings.
“We appreciate how difficult the wait is, especially for those within the community and beyond who have been impacted by the events in Keyham. When we are able to provide greater detail, we will do so.
"We are in contact with the bereaved families and those injured in the shootings to whom I again send my sympathies at this time. We appreciate their patience while our and other enquiries have been taking place."
Mr Ford added that Devon & Cornwall Police had co-operated fully with the investigation and they will await formal responses from them around the disciplinary matters and learning recommendations relating to firearms licensing.