Devon and Cornwall Police are facing a criminal investigation into alleged breaches of health and safety rules before the mass attack in Plymouth.
Jake Davison, 22, killed his mother Maxine, 51, after an argument before shooting four others in a 12-minute attack in the Keyham area of the city in August last year.
He then turned the shotgun on himself before armed officers arrived.
The shotgun and his license were returned weeks earlier by police, who had confiscated it in 2020 after Davison attacked two teenagers in a park.
The Independent Office for Police Conduct (IOPC) said on Tuesday: “We can confirm that we have launched an investigation into Devon and Cornwall Police into possible breaches of health and safety legislation in the operation of the firearms licensing unit prior to the mass shooting in August last year.
“On completion of our investigation into the force issuing a shotgun certificate and later returning a shotgun to Jake Davison, we sought expert legal advice and have since decided to launch a criminal investigation.
“Our investigation will look at whether the office of the chief constable of Devon and Cornwall Police, as a stand-alone body, may have committed offenses against the Health and Safety at Work etc Act 1974.”
Jim Nye, assistant chief constable of Devon and Cornwall Police, said: “The force co-operated fully with the IOPC investigation, the coronial process and commissioned an independent review of the force’s firearms licensing procedures.
“We are aware of the latest developments from the IOPC investigation and continue to co-operate fully with them while we consider the next steps the force may choose to take in this matter.
“The force notes that this development is in its early stages and no determination has been made regarding possible corporate culpability.”
Police sources stressed that no person faced a criminal investigation. A sole proprietorship is a legal entity consisting of a single office.
Three-year-old Sophie Martyn, her father Lee, 43, Stephen Washington, 59, and Kate Shepherd, 66, all died on the night of August 12 last year.
As part of the IOPC investigation, two members of staff have been served with serious misconduct notices, while one officer has been served with a misconduct notice.
At a preliminary inquest at Plymouth Coroner’s Court, Edward Pleeth, a barrister representing the IOPC, told senior coroner Ian Arrow the inquest would take some time but he believed it would not affect the inquests, which were due to start in January.
Jason Beer KC, representing the force, said he was not seeking to postpone inquiries in light of the development as the force was committed to providing immediate answers to key questions.
The social media use of Davison, an apprentice crane operator, suggested he held extreme misogynistic and homophobic views, as well as an interest in guns.
A further preliminary inquiry will take place on December 19.