All four victims of the massacre in the Alps - including a British man, his wife and an elderly relation - were shot twice in the head, it was disclosed yesterday.
French authorities said the findings of the post mortem examinations showed the assassins' "absolute determination to kill their victims".
The ruthlessness of the murders will fuel speculation that Saad al-Hilli and his wife Iqbal, who were on a caravanning holiday in the region, were victims of a professional hit. A cyclist who stumbled across the scene was also gunned down on a forest track near Lake Annecy on Wednesday afternoon.
Eric Maillaud, the prosecutor in the case, said the post mortem findings did not "necessarily" prove professional hit men were involved but added that, while he could not rule out the couple being killed by a madman, "it's not at the top of our list of possible theories".
"All four were killed by several bullets and all four were hit twice in the head," Mr Maillaud said, adding: "The whole scene was played out in a very, very short time."
He refused to say what type of weapons were used in the attack or how many there were.
There are several theories as to the motive for the attack, including the possibility that Mr Hilli and his family were victims of a carjacking that went wrong.
Mr Maillaud warned that speculation that an alleged financial dispute between the two brothers may have led to the killing was not based on any firm evidence.
"Everyone talks about a dispute between the brothers as it was an established fact. The brother says there was no dispute so let us remain cautious about that," said Mr Maillaud,
"He will be interviewed in the coming days, just as we will interview any other members of the [extended] family.
"It is no more and no less than that."
French police working alongside their British counterparts began searching the Hilli family home in Claygate, Surrey yesterday.
Five gendarmes led by Colonel Marc de Tarle have flown into Britain to work on the case, although not all were involved in the search.
Col de Tarle said yesterday: "This is an inquiry which is turning out to be long and complex." A French police source said the search would be "very long" and would go "beyond the day".
Speaking outside Woking police station in Surrey, Col de Tarle said: "We are here within the framework of a request for international mutual help launched by the French judicial authorities towards the British authorities.
"The co-operation is working well and the British police are putting in place all necessary means on a human and a technical level."
Forensics officers in protective overalls entered the house in the quiet commuter village in the search for clues to the killings.
Scene of crime officers erected a white tent outside the front door and - dressed in blue and white overalls, face masks, gloves and hair nets - began the process of sifting for evidence.
They also took in cameras while uniformed officers guarded the entrance gate.
Families have been visiting the home throughout the day to leave floral tributes outside the front of the house.
Along with bouquets of flowers, teddy bears had been left and a Mr Men Little Miss Trouble book.
In France, a group of 14 policemen spent time at the murder scene - at a remote car park, set off a steep and winding track above the village of Chevaline - to conduct further searches in the undergrowth.
"We're looking for anything that might be of use to the investigation," said one officer, who explained that they were still searching an area within a two-and-half mile (four-kilometre) radius from the car park.
Police, equipped with rucksacks, maps and hiking boots, said that they were not looking for a murder weapon, but for abandoned clothing or tyre tracks.
The Hillis' caravan was yesterday taken away in a low-loader from Le Solitaire du Lac campsite in the small town of Saint Jorioz, where the family had arrived on Monday.
Holidaymakers at the site said the family had appeared a happy one and that the couple's children, who both survived the attack, had picked apples from the tree by the caravan.
Before their belongings were taken away a child's pink bicycle could be seen propped up inside an awning while a children's DVD was placed on a window sill.
The hunt for the killers extended beyond the French borders yesterday with calls to Italian and Swiss authorities to search for a getaway car.
Witnesses have said they saw a green four-wheel-drive vehicle in the area at the time of the killings, and possibly a motorbike.
Mr Maillaud said: "In searching for the vehicle, of course, the Swiss and Italian police are involved in dealing with this."
Detectives are also piecing together the route taken by the Hillis as they drove through France. They left their home in Surrey at the end of August and arrived at the campsite on Monday.
Two mobile phones found in the car will be examined for clues as to who the family had been speaking and who knew where they were going.
Police will want to find out whether the couple stopped at a house in the Dordogne, reportedly owned by Mr Hilli.
"We think they did a tour around France, possibly down to the south. We're trying to piece together their movements," said Mr Maillaud,
Mr Maillaud said: "In my 25 years as a magistrate I have never had to deal with a crime this terrible.
"The most important thing for me is that, many years in the future, I can say to the girls: 'thanks to the work of the investigators, and their British colleagues, we have found those who killed your parents'."
Investigators will want to examine Mr Hilli's long professional career to see if it offers clues to what happened. He was a director of two companies, although the sums recorded in accounts did not seem significant.
He had also worked as a consultant for a number of hi-tech companies over the years and, according to one friend who wished to remain anonymous, had recently applied for a job abroad - not far from the campsite at Annecy.
Mr Maillaud said the Hillis' four-year-old daughter Zeena, who hid under her mother's skirt during the attack and survived physically unscathed, would only be interviewed once to avoid further psychological trauma. Her seven-year-old sister Zainab, who was badly beaten and shot once but survived, remains in a medically induced coma.
He said it was for doctors to decide when she would be brought out of the coma and questioned.
Two family members visited the children yesterday, accompanied by a British social worker, although French police refused to say who they were.
One possible area of inquiry will be the work carried out by Mr al-Hilli in his professional life. The 50-year-old was an engineer and, according to one friend, had recently applied for a job at the Cern laboratory over the border in Switzerland. The complex is a European-wide centre for nuclear research and home to the Large Hadron Collider.
A friend of Mr Hilli told The Sunday Telegraph: "Saad was a very talented mechanical engineer but it is hard to find work in our industry, however talented you are.
"Saad had asked me if I might be able to help him get a job at Cern.
"My old employer has connections there and I would have helped him if I could. I would have given him a glowing reference. As I understand it he did apply but I do not know if he had been accepted.
"By the time of his death, I had not been contacted by anyone for a reference."
Mr Maillaud said the search of the home in Surrey would help to build up a profile of the family, while stressing that there should be no presumption that they were involved in any activity that might have made them targets.
He said: "We will try to find out the maximum amount about the life of this family, about the father's professional life, the companies he may have worked for, as well as the history and heritage of the family."
Additional reporting by: Patrick Sawer and Richard Orange in Stockholm, Jason Lewis, Ben Leach, Julie Henry, Josie Ensor, Matt Sandy and Imogen Adie in London, Lubna Naji in Baghdad and Philip Sherwell in New York
The Sunday Telegraph, London