Police confiscated but then returned a shotgun after an assault claim last year against a man who murdered five people in Plymouth on Thursday.
The Independent Office for Police Conduct (IOPC) confirmed late on Friday that Devon and Cornwall Police had suspended Jake Davison's shotgun certificate and taken the weapon from him after he was accused of assault in September 2020.
But the force reinstated the permit and gave him back his shotgun in December of that year.
IOPC Regional Director David Ford said they would investigate that decision after receiving a "mandatory referral" from Devon and Cornwall Police.
"After assessment of the referral we have determined we will carry out an independent investigation focusing on Jake Davison's firearms licensing history and its impact on the tragic events of Thursday 12 August", Ford said.
"We will examine what police actions were taken and when, the rationale behind police decision-making, and whether relevant law, policy and procedures were followed concerning Mr Davison’s possession of a shotgun".
Ford said the probe would ask whether the force had information of 22-year-old Davison's mental health — and if that was taken into consideration before returning his gun.
He stressed that the "prompt" police response to the shooting incident itself was not under investigation, but the probe would determine "any causal link between the arrival of police and Mr Davison apparently shooting himself".
Davison's rampage on his home street of Biddick Drive in the Keyham district around 6pm on Thursday evening left six people dead, including his 51-year-old mother Maxine Davison, three-year-old girl Sophie Martyn, and Davison himself.
Devon and Cornwall Police named all the victims on Friday, including the girl's 43-year-old father Lee Martyn, Stephen Washington, 59, and Kate Shepherd, 66, who was injured and died later at the city's Derriford Hospital.
Local MP Luke Pollard said there were "valid questions that need answers" over the shooting, but asked residents to "give the Police the space to do their work".
Davison worked for Babcock, the defence and aerospace company that owns the Devonport military dockyard next to Keyham — where the Royal Navy's nuclear-powered submarines are serviced and decommissioned, along with many of its surface ships.
HMS Trenchant sailed into Devonport for the last time today before being de-commissioned later this year.
Few Historical Precedents
Thursday's massacre was the sixth US-style shooting in mainland British history, the two most notorious of which prompted major restrictions on firearms ownership.
Barry Williams, later known as Harry Street, killed five people and wounded four in his four-hour 1978 shooting spree in the West Midlands towns of West Bromwich and Nuneaton with two handguns.
The 1987, the Hungerford massacre saw Michael Ryan shoot dead 16 people — including his own mother and an unarmed police officer — and injured 15 with two semi-automatic rifles and a Beretta handgun, before taking his own life after being surrounded by police in the Berkshire town. The next year, Parliament outlawed semi-automatic rifles above .22 rimfire calibre and placed restrictions on shotguns with a capacity of more than three rounds.
Two years later, 22-year-old Robert Sartin killed one man in the Tyne and Wear town of Monkseaton, and injured 14 other people in a rampage with his father's double-barrelled shotgun that lasted just 20 minutes before his arrest.
In 1996, Thomas Hamilton, armed with two 9mm semi-automatic pistols and two .357 magnum revolvers and over 700 rounds of ammunition, shot dead 16 pupils and one teacher at Dunblane primary school near Stirling in Scotland before shooting himself. 15 others were injured. Future Wimbledon tennis champion Andy Murray, who later recalled attending a youth group run by Hamilton, survived the massacre unharmed along with his brother. The Conservative government legislated to ban all modern handguns over rimfire calibre in the wake of the shooting, with Labour Prime Minister Tony Blair's MPs later extending that to small-calibre pistols.
The next British mass shooting was not until 2010, when taxi driver Derrick Bird killed 12 people and injured 11 others in a shooting spree across the north-western county of Cumbria, before committing suicide. Bird was armed with a .22-calibre rifle and a shotgun, of which he had sawn down the barrel at home.