A French man from Surrey Saad al-Hilli and his wife Iqbal on Wednesday were found dead inside BMW near Lake Annecy in the French Alps.
The French police have been trying to seek the motive behind the shooting, involving the death of three family members.
Their daughters, aged four, and seven hid inside the car for about eight hours and were eventually severely injured.
French prosecutor Eric Maillaud said three of the four victims of the killings were shot in the head.
A French cyclist, Sylvain Mollier, 45, was also found shot dead nearby.
Neighbors of the family in Claygate, Surrey, named the wife as Iqbal, the elder daughter as Zainab and the younger daughter as Zeena. Mr al-Hilli has not yet been officially named by the French authorities.
Zainab was in a medically induced coma in Grenoble University Hospital after being shot once and suffering head injuries. Both girls are under police protection in hospital.
Maillaud said the motive for the attack, in Chevaline, remained a mystery.
“I won’t say it was professional, what I will say is it was tremendous savagery. And what is certain is that somebody wanted to kill,” he added.
An automatic pistol was used, and the killer “targeted” the victims rather than indiscriminately firing into the car.
Local police said a British cyclist, who had served in the RAF, found the adults and a child on a forest road. The younger daughter was concealed beneath her mother in the vehicle.
Police in Surrey said they were working with the French authorities and the Foreign and Commonwealth Office.
The family had arrived on holiday at the nearby Le Solitaire du Lac campsite in Saint-Jorioz on Monday and had been due to leave at the end of the week.
French president, Francois Hollande said the authorities would “do our utmost to find the perpetrators”.
UK Prime Minister David Cameron said: “Obviously the faster we can get to the bottom of what happened, the better.”
The British ambassador to France, Sir Peter Ricketts, described it as a “terrible, tragic event, a brutal murder”.
Reports say the investigations are still in its early stages, there are conflicting theories emerging as to the motive.
