A man who killed three women then turned the gun on himself argued with his victims at his home just before he shot them, police said.
It has also emerged Michael Atherton had his legally-owned guns returned to him after police removed them in 2008.
The 42-year-old was found dead at the house in Horden, near Peterlee, County Durham, on Sunday.
His partner, Susan McGoldrick, 47, her sister Alison Turnbull, 44, and niece Tanya Turnbull, 24, were also shot.
Their bodies were found in different rooms in the semi-detached property in Greenside Avenue.
A post-mortem examination has revealed they died from shotgun injuries.
Police confirmed the weapon used was a shotgun that was legally registered to Mr Atherton as part of his shotgun certificate.
A Durham Police spokesman said that in the hours leading up to the shootings, Mr Atherton had been out drinking locally.
His partner was out drinking separately with a group of friends, which included her sister and niece.
Det Supt Paul Goundry said: "We believe Mr Atherton and Susan returned to their home separately and shortly afterwards an argument took place.
"Mr Atherton produced a shotgun and in a matter of moments shot dead Susan, Tanya and Alison before turning the gun on himself."
Three other people managed to escape, including a 19-year-old woman - named locally as Laura McGoldrick - who escaped from an upstairs window and suffered minor injuries.
Mr Atherton was licensed to own six weapons, three of them shotguns and a further three "section-one" firearms.
Police confirmed the force took away his guns following an incident in 2008, but they were later returned.
A spokesman said that decision was in line with national procedure.
"It was one person's word against another," he said.
"If there was no grounds for applying for the revocation of a firearms licence the guns would be returned to the firearms licence holder."