A British man is believed to have killed three female family members and then taken his own life at a house in County Durham in northern England, local police said on Monday.

Police said they found the four bodies in different rooms in the house with gunshot wounds, and a shotgun was found next to the man's body.

Detective Superintendent Paul Goundry said he believed "the shootings were carried out by the 40-year-old male" identified as Michael Atherton, a taxi driver.

Police said all four victims were related or known to each other. They also confirmed there were other people in the house before the shootings but said they are now treated as witnesses.

A teenage girl, suffering minor injuries, escaped through an upstairs window and went to neighbors for help.

At the news conference, Assistant Chief Constable Michael Banks said a resident of the house held a firearms license.

"I can confirm that a resident at that address was the lawful holder of shotgun and firearms licenses and we cannot confirm at this stage whether any of those weapons that were lawfully held have been used in this," he said.

Local police were alerted around midnight on Sunday when receiving phone calls to report the shootings. Police investigation is still in progress.