- Video report by ITV News Correspondent Sejal Karial
A woman and her daughter would still be alive had police not returned their killer's shotguns and firearms certificate to him just months before, according to a devastated relative.
Surrey Police seized 82-year-old John Lowe's weapons after he threatened to shoot partner Christine Lee, 66.
But they were eventually returned to him, and Lowe went on to kill Ms Lee and her 40-year-old daughter Lucy at his puppy farm near Farnham, Surrey, on February 23, 2014.
Now the Independent Police Complaints Commission (IPCC) has released a critical 76-page report into Surrey Police's handling of the case.
And another daughter of Ms Lee, Stacy Banner, has also slammed the force, saying her mother and sister could still be here today.
Lowe had only had his shotguns seized and firearms licence temporarily revoked seven months before he embarked on his deadly rampage.
Mrs Banner believes that Surrey Police did not act properly by returning the weapons to him - and now plans to sue the force.
The IPCC said Surrey Police failed to consider and properly assess information relating to Lowe before returning the firearms to him.
And the police watchdog found the force's firearms licensing team lacked training and was staffed by people failing to do their duties properly.
Prosecutors have ruled out criminal charges against police staff, but said two firearms licensing team members have cases to answer for misconduct.
In light of the report, Mrs Banner said: "It is devastating to see your worst fears confirmed in black and white about how those entrusted with the public safety can abuse and neglect their powers.
But for the police's failings, my mum and sister would be here today."
The IPCC found evidence that during the license review, the two officers did not take simple investigative steps such as accessing Surrey Police's information and intelligence systems.
The pair also did not highlight risks posed by Lowe or properly address whether he was a danger to the public when deciding to hand back his shotguns and certificate.
IPCC associate commissioner Tom Milsom said: "Our investigation paints a deeply concerning portrait of how Surrey Police's firearms licensing team operated at that time."
The IPCC also carried out a separate inquiry into how family liaison officers dealt with Mrs Banner in the aftermath of the killings, and her subsequent arrest for which no further action was taken.
Investigators found a detective constable and a detective sergeant had cases to answer for misconduct, and a detective inspector had a case to answer for gross misconduct.
Lawyers for Mrs Banner said: "The IPCC recommendations in this shocking case include that the licensing team should liaise with officers investigating allegations against firearms license holders."
Responding to the IPCC report, Surrey's assistant chief constable Helen Collins said that changes were "quickly made" in light of the killings.
Lowe was ordered to serve a minimum of 25 years for the two murders at Guildford Crown Court in October 2014.