A mother and son were hacked to death in their Banbury 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 Swindon, 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 by police in the property in Mold Crescent, Banbury, 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."
Det Insp Duffy added: "We 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.
He said: "We would like to reassure the community of Banbury that this is being treated as an isolated incident and we believe the suspect we have in custody is an ex-partner of the woman who was killed.
"A scene watch is still in place and we have neighbourhood officers delivering information to local residents to inform them of what has taken place. We ask anyone who lives in the area and has any concerns to contact Thames Valley Police.
"At this stage we cannot confirm the exact nature of the injuries the man and woman sustained, but we can confirm that some kind of weapon was used during the attack.
"At this stage it has yet to be recovered but we have a team of detectives working on the case.
"We would ask anyone who has information that could assist police enquiries to contact the Thames Valley Police Enquiry Centre on 0845 8 505 505.
"If you don’t want to speak to police and don’t want to leave your name contact the Crimestoppers charity on 0800 555 111.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article