A gunman killed three relatives and a neighbour in a late-night rampage in a Swiss village before turning his weapon on himself.
The killer, a 36-year-old Swiss national who was separated from his wife and the couple's three children, shot dead his father-in-law, mother-in-law and brother-in-law, police told a news conference.
The killings took place late on Saturday in and around a residential building in Wuerenlingen, a community of about 4,500 people north-west of Zurich.
Police said the gunman lived in another part of Switzerland and was known to police for violent behaviour.
The murder weapon was unregistered, they said.
A neighbour, who was not identified, told Swiss newspaper 'Blick' that she heard four shots in succession fired very quickly, then two further shots after a pause. She had heard no screams.
"We first found the bodies of two men, respectively 58 and 32, then the police found the body of a woman of 57 years," the head of the Aargau Police, Markus Gisin, told a press conference.
"All the victims were in the same address. All were adults, and the gunman is thought to be among them.
"He is thought to have turned his gun on himself," said Mr Gisin.
The neighbour was shot after he had come to investigate.
The gunman, from the town of Schwyz, had been separated from his wife and three children.
Detectives investigating the murders found that he had used a non-military weapon. Canton (state) Aargau police spokesman Bernard Graser said the motive was a family dispute and the case is not terror-related.
"This is an exceptionally terrible crime, atypical for such a tranquil residential area," he told '20 Minuten', but stressed that children were "at no time in danger".
The incident took place just a few hundred metres from the scene of another multiple murder that rocked the small town 30 years ago.
On December 13, 1985, a 33-year-old estate agent called Alfredo Lardelli shot two prostitutes and a love rival to death.