French police guard at the crime scene
French prosecutors say a possible family dispute over money is one of their lines of inquiry into the killings of four people in the Alps.
Three members of a UK family - named as Saad al-Hilli, 50, from Surrey, his wife and mother-in-law - were shot dead near Lake Annecy on Wednesday.
Police hope to speak later to al-Hilli's four-year-old daughter, who was found hiding at the scene, reports the BBC.
A French cyclist thought to have been a witness was found shot dead nearby.
He has been named as 45-year-old Sylvain Mollier.
Prosecutor Eric Maillaud told AFP news agency that one lead being followed was a possible dispute between brothers over money.
"It seems that there was a dispute between the two brothers about money. This seems to be credible information coming from the British police," he said.
"The brother will have to be questioned at length. Every lead will be meticulously followed."
Another daughter, seven, 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.
Neighbours of the family in Claygate, Surrey, named the wife as Iqbal, the elder daughter as Zainab and the younger daughter as Zeena. al-Hilli, who was originally from Iraq, has not yet been officially named by the French authorities.
The younger girl had spent about eight hours hiding in the car with the bodies of her parents before being discovered by officers.
Maillaud said three of the four victims of the killings had been shot in the head, and that 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 said.
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. They said there were signs of a vehicle braking at the scene.
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".