Police found four adults shot dead at a house in Newcastle, northern England, after a man walked into a police station carrying a bag containing guns, Northumbria Police said.
Officers arrested the man, who they believe is closely related to the victims, after he said his bag also contained a home-made nail bomb.
The station was evacuated and bomb disposal experts carried out a controlled explosion on the device.
The victims, a man and a woman in their 70s and two men in their 40s, were found at the house, about half a mile from the police station.
All are thought to be members of the same family. They were found fully dressed and had been shot at close range.
Police said their bodies could have been in the house for some time as there were no reports of shots being heard.
The arrested man, aged 40, has a history of mental illness and may have a military background, police said.
He had walked into a police station at around 6:15am and told officers about the bodies.
Police said his bag contained a handgun with a silencer, a pump-action shotgun and an amount of ammunition. They said they were not seeking anyone else in connection with the inciden