The police watchdog is investigating why the man who shot dead five people in Plymouth on Thursday had been given back his confiscated gun and permit.
Jake Davison was stripped of the weapon and its licence in December after he was accused of assault, the Independent Office for Police Conduct (IOPC) says.
It is to examine why Devon and Cornwall Police decided to return them in July.
Meanwhile, hundreds of people attended a vigil on Friday evening close to where the shootings took place.
Davison, 22, shot his 51-year-old mother Maxine Davison, also known as Maxine Chapman, at a house in Biddick Drive before he went into the street and shot dead Sophie Martyn, aged three, and her father Lee Martyn, aged 43.
He killed Stephen Washington, 59, in a nearby park, before shooting Kate Shepherd, 66, on Henderson Place. She later died at Derriford Hospital.
Davison also aimed and shot at two local residents – a man aged 33 and a 53-year-old woman – who are known to each other, in Biddick Drive.
His attack, in Keyham, lasted about six minutes before he turned the gun, a pump action shotgun, on himself.
Prior to the shooting, Davison had posted hate-filled online rants about single mothers and about his own mother in particular, calling her “vile, dysfunctional and chaotic”.
The IOPC said it had launched an investigation on Friday evening after it received preliminary information that Davison’s firearm and licence were returned to him in early July this year.
The certificate and shotgun had been removed from him by police in December 2020 following an allegation of assault in September 2020, the watchdog said.
Prime Minister Boris Johnson had earlier said the issue of how Davison came to legally own a gun should be “properly investigated”, adding that the shooting was an “absolutely appalling” incident.
Regional IOPC director David 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.”
It will also look at whether the force had information concerning Davison’s mental health and if this was “appropriately considered”.
Chief Constable Shaun Sawyer told Sky News that those present had faced “some of the most challenging scenes”.
“We’ve never in my time had homicide followed by a rampaging firearms attack on random members of the public and then taking one’s life,” Mr Sawyer said.
“That is without precedent in my time as chief constable.”
Police are not treating incident, the worst mass shooting in Britain since 2010, as terror-related.
However, Davison made references to “incels” in some online social media videos.
Incels are members of misogynistic online groups of “involuntary celibate” men, who blame women for their sexual failings and who have been linked to a number of violent acts around the world.
Officers are continuing to investigate 13 scenes within a cordon that is expected to remain in place throughout much of the weekend.
On Friday evening, hundreds of people placed flowers and candles in North Down Crescent Park.
One note read: “Rest in peace. Thoughts and prayers with you all. A tragic loss. Such beautiful souls. Love to all.”
Plymouth Sutton and Devonport MP Luke Pollard tweeted images of the vigil and the victims’ names, writing: “Tonight our community came together to remember the victims of the shooting in Keyham.”
Flags in Plymouth are being flown at half-mast, while Smeaton’s Tower was lit up as a mark of respect to the victims on Friday night.
