Three men and a woman from the same family have been found shot dead at a house in Newcastle.
Army bomb disposal experts carried out a controlled explosion
Officers found a couple in their 70s and two men in their 40s in Benwell Grove after a 40-year-old man went into Westgate police station on Sunday.
A controlled explosion was carried out at the police station after the man claimed he had a home-made nail bomb.
The man, who has a history of mental illness, is thought to be a "close relative" of those killed.
Detectives are not looking for anyone else in connection with the inquiry and have stressed it is not a terrorist-related incident.
Roads around the scene remain closed as investigations continue.
The man entered Westgate police station - which is only yards from Benwell Grove - at about 0615 BST.
He had a bag containing a handgun with a silencer, a pump-action shotgun, a quantity of ammunition and an improvised explosive device, described by police as a "thunder flash with nails around it".
Shortly afterwards officers found four bodies in two downstairs rooms of the house.
The man in custody is thought to be a former soldier.
Asst Ch Con Keith Matheson, of Northumbria Police, said the victims had suffered fatal gunshot wounds at close range.
He said there were no signs of a struggle and all were fully clothed.
He said: "We believe he has a mental illness. The extent of that mental illness we've still to clarify, but clearly that's a line of inquiry the investigatory team are carrying out.
"At this stage it is not possible to say how the firearms came to be in possession of the man in custody.
"There was believed to be an improvised explosive device in the holdall.
"That was essentially believed to be a thunder flash device with nails tied around it and we were informed of that by the male concerned."
The police station was subsequently evacuated and an Army bomb disposal team carried out a controlled explosion on the holdall.
Police officers inspect the scene at the Benwell Grove house
Mr Matheson added: "It would appear that those who died were closely related to the man who's in custody."
Police added that the man was not believed to be a licensed firearms holder.
Residents close to the scene were being moved to a nearby school.
The Benwell Grove area is in the west end of Newcastle and is mainly residential, with some neighbourhood shops.
Westgate Road, which is a major route into the city, has been closed between its junctions with Brighton Grove and Grainger Park Road.
The city's General Hospital remained open as normal.