A woman killed along with her son in Oxfordshire died from "multiple head injuries consistent with a heavy bladed instrument", post-mortem tests showed.
The pair, named locally as Sally Cox, 43, and Martin Faulkner, 22, were attacked with an axe on Monday.
Mrs Cox's daughter, Amy, 19, who was injured when she jumped out of a window at the house in Banbury with her sister Katie, 13, remains in hospital.
Michael Kelly, 45, has been arrested on suspicion of murder.
Mr Kelly, who is believed to be Mrs Cox's ex-partner, was arrested in Swindon, Wiltshire, by police a few hours after the two bodies were found.
A post-mortem examination on Mr Faulkner was carried out earlier but results have not yet been made public.
The bodies were discovered at Mrs Cox's end-terraced home in Mold Crescent, which was described by officers as "a horrific scene".
Det Insp Steve Duffy, of Thames Valley Police, said police had received no previous calls to the address about domestic incidents.