Plymouth gunman Jake Davison was allowed to keep his firearm despite admitting assaulting two youths in a park.
Police did not seize his shotgun and licence as he had been offered a place on their Pathfinder scheme after the attack last September.
The intervention programme is meant to be an alternative to being charged or cautioned.
But at the end of November last year, a scheme worker raised concerns with the force’s firearms licensing department about Davison’s possession of a shotgun.
The gun and certificate were then seized by police on December 7.
But following his completion of the scheme in March this year, and a subsequent review by the firearms licensing department, the shotgun and certificate were returned on July 9.
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On August 12, Davison killed his mother Maxine, 51, Stephen Washington, 59, Kate Shepherd, 66, Lee Martyn, 43, and three-year-old Sophie Martyn, before turning the gun on himself.
Police revealed he had been issued a gun licence for clay pigeon shooting.
The Independent Office for Police Conduct is investigating Devon and Cornwall Police’s decision to return the licence and gun.
Chief Constable Shaun Sawyer said he had not been asked to step down, but would reflect on his position “once we have the facts”.
“As a man and a father and a parent, a child has lost its life on my watch, through a mechanism that is licensed by me,” he said.
“I understand the unease. This was a licensed firearm, I absolutely accept that.
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"I’d ask people to reflect that those firearms, in my 10 years as chief constable, have never been used in that way.
"This is unprecedented for us in Devon and Cornwall and the city of Plymouth.”
However, Mr Sawyer said firearms were “part of a rural economy and we have to accept that”.
Mr Sawyer said he believed new guidance from the Home Office would see social media checks become a “must” for forces.
All forces across England and Wales have been asked by the Government to review their firearms processes after the killings.
An IOPC spokesman said: “The investigation will consider what background checks were made by the police, including from open source material, and whether the force had any information, from Mr Davison’s GP and any other mental health services concerning his state of mind.
“It will also look at the force‘s decision to divert Mr Davison from prosecution for the assaults last year.”
