A mother and son were hacked to death in their own home by a man neighbours said was wielding an axe.
The woman's teenage daughter was badly hurt in the attack and her 13-year-old sister "severely traumatised", police said.
Armed police arrested a 43-year-old man a few hours later in Wiltshire. Police said the suspect was the woman's former partner.
Detectives and forensic investigators found a "horrific" scene when they arrived at the terrace home this morning.
The mother, in her 40s, was discovered in the property in Mold Crescent, Banbury, Oxfordshire, by police at 6.45am, along with her son, who was in his 20s.
The victims were apparently murdered inside the house, but two of the woman's daughters managed to flee the scene.
A girl in her late teens was badly injured and her 13-year-old sister "traumatised" escaped uninjured.
Detective Inspector Steve Duffy confirmed the murder victims were believed to be mother and son and said of the surviving sisters: "They have obviously been through a horrific ordeal this morning."
Mr Duffy added: "As you can appreciate, officers went to the address and were faced with a horrific scene.
"There were two people obviously dead or very close to death at the scene.
"All I can confirm is their injuries are consistent with blunt trauma and I can't say if an axe was used but a firearm was not used.
"The police traced a suspect to an address in Wiltshire and police went to that address and effected an arrest."
The policeman said there was a "background" to the deaths and they were not random attacks.
The two survivors will be interviewed by detectives later today, if they can face it.
Mr Duffy said police had received no previous calls to the address regarding domestic incidents.
The victims have not been formally identified and post-mortem examinations will be held later today to establish cause of death.
A shocked neighbour of the victims, who did not want to be named, said: "All we've heard is there's been a murder and an axe was involved.
"I know the house but I don't know who lives there.
"We can't get up there because the police are blocking it. They've stopped the cars going up and down.
"It's a very quiet street really. I never hear of any trouble."
The family are believed to have lived in the house for a year, Mr Duffy said.
More than 40 police officers were working on the investigation this afternoon.
Ten uniformed officers remained at the scene, along with five detectives and a number of forensic investigators.
The end-terrace house remained cordoned off, and a long stretch of the road was closed to traffic and pedestrians in both directions.
Police officers were also present at a second house in Mold Crescent, opposite the scene of the attack.Reuse content