Detectives investigating the New Year's Day gun massacre in Peterlee in County Durham are due to probe how the alleged killer managed to hold various gun licences.
Taxi driver Michael Atherton, 42, is understood to have shot and killed his partner Susan McGoldrick, 47, her sister Alison Turnbull, 44, and her niece Tanya Turnbull, 24, before turning the gun on himself at their house in Horden, Peterlee. Three other women, including a teenager, managed to escape from the scene.
Atherton, 42, held various gun licences despite having a previous history of threatening to harm himself. The incident, which had been reported to the police, occurred three years ago.
The Independent Police Complaints Commission has been asked to probe the killings.
Police have confirmed that Atherton held at least six gun licences. Michael Banks, assistant chief constable, said: "I can confirm that a resident at that address was the lawful holder of shotgun and firearms licences and we cannot confirm at this stage whether any of those weapons that were lawfully held have been used."