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.
A French cyclist thought to have been a witness was found shot dead nearby.
The BBC understands there was a dispute between Mr al-Hilli and his brother over inheritance from their parents.
This involved a house in Spain left by the men's father, who died last year.
A family friend has also told BBC News of tension with Mr al-Hilli's brother, Zaid, after the family home in Claygate, Surrey - which had apparently been in their mother's name - was left to Saad.
Speaking to AFP news agency, prosecutor Eric Maillaud said: "It seems that there was a dispute between the two brothers about money. This seems to be credible information coming from the British police.
"The brother will have to be questioned at length. Every lead will be meticulously followed."
Post-mortem examinations are due to be carried out on the four victims' bodies on Friday.
And for the first time since the shootings, French police have lifted a road block some two miles from the site - where blood-stained pebbles, tyre marks and small shards of glass could still be seen.
A dent at the back of the car park where the family's BMW estate hit a bank of dirt during the attack was also visible.
Police hope to question Mr al-Hilli's four-year-old daughter, who spent about eight hours hiding in the car with the bodies of her parents before being discovered by officers at midnight on Wednesday.
Another daughter, aged eight, 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 in Claygate named the wife as Iqbal, the elder daughter as Zainab and the younger daughter as Zeena. Mr al-Hilli, who was originally from Iraq, has not yet been officially named by the French authorities.
The cyclist has been named as 45-year-old Sylvain Mollier.
Mr 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.
The family friend said that the al-Hilli family had fled Iraq in the 1970s because they were seen as opponents of Saddam Hussain.
But he added that Saad al-Hilli was "not a man who made enemies", describing him as devoted to his family.
Local French police said a British cyclist, who had served in the RAF, found the adults and a child on a forest road. 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".