An investigation is to be launched into how a man who shot dead his partner and two other women before turning the gun on himself was allowed to hold licences for shotguns and other firearms.
Michael Atherton, 42, killed his 47-year-old partner, Susan McGoldrick, her sister, Alison Turnbull, 44, and her niece, Tanya Turnbull, 24, before taking his own life on New Year's Day.
The four bodies were found in the semi-detached house in Greenside Avenue, Horden, Peterlee, after police were alerted to shots being fired shortly before 11.45pm on Sunday night.
Police maintained a wide cordon around Greenside Avenue on Tuesday. Forensic specialists were seen arriving in a white van and uniformed officers kept guard at the top of the road.
As the shooting began, a teenager, understood to be McGoldrick's 19-year-old daughter, Laura, fled through an upstairs window and raised the alarm.
Durham Police Assistant Chief Constable Michael Banks told a news conference yesterday that a resident at the address was the "lawful holder" of shotgun and firearms licences.
He said these were for three shotguns and three "Section One" firearms, a category requiring greater authorisation than a shotgun licence.
The revelation that Atherton held licences for shotguns and other firearms is likely to spark renewed debate over whether to further tighten Britain's gun licensing laws.
Police could not confirm at this stage whether any of those lawfully held weapons had been used in the shooting, Mr Banks said.
He added that it was believed Atherton may have been a member of a gun club but police were still investigating this.
Home Office figures for England and Wales show there were 141,775 certificates on issue for Section One firearms and 580,653 for shotguns at the end of March 2010.
Police revoked 302 firearms licences and 1,076 shotgun licences in 2009-10.
In December 2010, the Commons Home Affairs Select Committee described legislation in England and Wales as a "complex and confused" mess and called for tighter restrictions on gun licences.
The MPs called for criminals who receive suspended jail terms - like taxi driver Derrick Bird, who killed 12 people in the 2010 Cumbria shooting spree - to be stripped of firearms licences.
The British Association for Shooting and Conservation (BASC) warned against "any knee-jerk reaction" to the tragedy.
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The organisation expressed sadness and offered its sympathy to the victims but added: "No conclusions can be drawn from this case until the full facts are known.
It said: "There has just been a comprehensive parliamentary review of firearms law in the UK and the facts in this incident need to be firmly established."
Police have also revealed that they had had "minor contact" with the family in the past. In 2008 police were contacted after claims that Atherton was threatening to harm himself.
The matter has been voluntarily referred to the Independent Police Complaints Commission (IPCC) by Durham Police, as the force had previous contact with the family.
A spokeswoman for Durham Police said last night: "Police are continuing to try and establish the exact circumstances of what took place and are speaking to family, friends and associates of those who died.
"However, we can confirm that police currently believe Mr Atherton shot the three women before turning the weapon on himself.
"The exact weapon used has still not been established and will also form part of continuing inquiries."
Police believe family members had been out at a pub or for a meal before returning to the house where the incident took place.
Post-mortem examinations were being carried out last night, and police hoped to release more details today.
The families of those involved have asked not to be contacted by the media. Some of the victims' relatives are also being treated as significant witnesses.
Senior investigating officer Detective Superintendent Paul Goundry said there had been other people in the house who were now being interviewed.
According to reports, McGoldrick's brother, Bobby Hardman, said the family was "numb from head to toe", adding: "We've lost three family members in one swoop and that's all I can say really."
Family friend Steve Patterson reportedly said Atherton's brother, Chris, had told him before the shootings that his brother had been suffering from depression.
Patterson was quoted in reports as saying: "He said he was fed up with the way his life was going. I think he started feeling down after an operation he had on his heart."