A woman who was murdered with her son died from "multiple head injuries consistent with a heavy bladed instrument", a post-mortem examination revealed today.
Sally Cox, 43, and Martin Faulkner, 22, who were named by neighbours, were butchered at their home in Banbury, Oxfordshire, early yesterday.
Neighbours said the pair were killed with an axe, and hours later detectives arrested a 45-year-old man who is said to be Mrs Cox's ex-partner.
Police described the murder scene as "horrific".
Today, a Thames Valley Police spokesman said: "A post-mortem on the woman has concluded she died from multiple head injuries consistent with a heavy bladed instrument.
"The post-mortem on the body of a 22-year-old man will take place today.
"Formal identification of both bodies will take place after that, hopefully later today."
The pair were apparently killed in the house and Mrs Cox's daughters, aged 13 and 19, managed to escape.
The older sister, believed to be called Amy, was badly injured and the 13-year-old, thought to be Katie, was uninjured but "severely traumatised".
The police spokesman added: "A 19-year-old woman is being treated in John Radcliffe Hospital where she is in a serious but stable condition."
Detectives have until 6pm this evening to charge the man.
If he has not been charged by then he will be released or police can detain him further with a 12-hour extension from a superintendent or more senior officer.
Yesterday police and white-suited forensics officers pored over evidence at the house in Mold Crescent.
A few hours later the suspect was arrested by armed police at a property in Wiltshire.
Today neighbours in Hatch Road, Stratton St Margaret, Swindon, where the man was apparently arrested, relived the scene.
One, who did not want to be named, said: "I went out to the doctors at about 11.10am, as I left I saw about eight armed policemen with guns, flak jackets, helmets, the lot, just outside the house.
"One of the policemen was in the right upstairs window - as you look at the house - shouting down to those below.
"I knew it was something serious as there were vehicles right up the street.
"I came back about 15 minutes later but they looked like they'd stood down by then.
"There was one chap who must have been the boss who was on the radio.
"I heard him say there was one person in the house - who must be the one they arrested - and three rooms were being rented out.
"They were still there when I went out again to work at about 1pm.
"When I came back at 4pm the armed police had gone but there were crime scene investigators. There has been a police car outside all night.
"There was a family living there with girls who used to hang around the garage.
"The guy had a pick up truck which always had branches in - he may have been a landscape gardener or something - I imagine it's him (who they've arrested), but I can't be sure.
"The family left a while back, but he had stayed on.
"This is a really nice area; you don't get rowdy behaviour, no hassle or anything.
"When I saw the police I thought it might be a drugs raid or a terrorist, you don't think it's going to be murder.
"You just don't know people, you think you know your neighbours but you just don't.
"It's not safe anywhere any more - you don't feel like it is anyway."
Another neighbour added: "A load of police turned up and bashed their way into the house.
"There were a load of marked and unmarked police cars right up the road.
"The house has been rented out several times. I've been here about six years and people come and go, so I don't take too much notice of them to be honest.
"He seemed to be a bit of an odd job man - he was always loading and unloading his pick up truck early in the morning and late at night.
"It's a quiet street, but it's just a sign of this day and age - nothing shocks me any more."