A 21-year-old man was being questioned by detectives after a knifeman killed two women and left a third injured at a house.
Police called to a cul-de-sac at Ireton Court in Thame, Oxfordshire, at 2.21am on Sunday found the pair with fatal injuries.
A third woman was taken to John Radcliffe Hospital in Oxford but her condition is not thought to be life-threatening, Thames Valley Police said.
According to reports, the two dead women were a mother and daughter aged in their 50s and 20s who were declared dead at the scene.
Police refused to confirm their relationship but said all parties were known to each other and no one else was being sought over the killings.
The suspect was detained by officers in Thame Market Place shortly after the women were found in the red-brick house. He was being quizzed at Abingdon police station.
Detective Chief Inspector Joe Kidman, the senior investigating officer, said: "We are in the early stages of our investigation.
"We have arrested a man on suspicion of murder. At this time it appears that all parties are known to each other and we are not seeking anyone else in connection with this case.
"We are working hard to establish the full circumstances of this incident and what led to the tragic deaths of these two women."
Formal identification and post-mortem examinations will not be carried out until further relatives have been informed.
