The family were on a camping holiday on the shores of Lake Annecy when they were attacked.
Eric Maillaud said the inquiry into the shootings in Annecy in early September was a "long task".
Saad al-Hilli, 50, was shot with his wife Iqbal, 47, his mother-in-law and a cyclist near Annecy.
BBC: French Alps murders: Uncle's concerns over police probe
The design student was reportedly taken to hospital in nearby Annecy, before being transferred to a specialist unit.
The family, from Surrey, were on a camping holiday on the shores of Lake Annecy when they were killed.
BBC: French Alps murders: Uncle's concerns over police probe
Ms Watts, also known as Mimi, was taken to a hospital in Annecy and placed on a life support machine.
BBC: Snowboarder Emily Watts dies five days after Alps crash
"Zambezia" had its world premiere at France's Annecy Animation Festival last month and was released in Israel earlier in July.
Prosecutor Eric Maillaud said the family was on a camping holiday on the edge of Lake Annecy in the Haute-Savoie region.
The 4x4 was being driven on the Combre d'Ire Road in Chevaline, near Annecy, about 20 minutes before the shooting, the force said.
Saad al-Hilli, 50, from Claygate in Surrey, was shot dead along with his wife, mother-in-law and a cyclist near Chevaline, Lake Annecy, on 5 September.
BBC: Alps al-Hilli family murders: Man charged in fraud inquiry
Mr al-Hilli's wife, Iqbal, and her 74-year-old mother who held a Swedish passport, were killed, close to the tourist destination of Lake Annecy, on Wednesday.
Perched above Lake Annecy, Chalet la Pricaz specialises in organic produce.
In Annecy on Saturday, Mr Maillaud said post-mortem examinations found the victims were killed by several bullets and "all four were hit twice in the head".
Now the contracts are coming up thick and fast, and Clear Channel has taken a number of important French cities, such as Montpellier, Nanterre and Annecy.
Ms Watts, a design student, was reportedly taken to hospital in nearby Annecy following the accident in the Grand Montets skiing area and then transferred to a specialist unit.
Using sterile swab kits, forensic scientists in cases similar to the one near Annecy would be expected to take samples from the car door handles, the steering wheel, the mirrors.
BBC: French shootings: What happens in the aftermath of murder?
The family - including Mr al-Hilli's dentist wife Iqbal, 47, and her mother, were murdered in their car while on holiday near Chevaline, close to the tourist destination of Lake Annecy.
He won the 100m in Annecy and was also a member of the triumphant 4x100m squad, helping the side achieve a narrow overall victory and qualify for the IAAF World Cup in Madrid.
Kuske actually started his sporting life as a track and field athlete of some note, claiming a bronze in Germany's 4x100m relay squad at the 1998 world junior championships in Annecy in France.
Local journalists said Mr Saunders was thought to have made a call by mobile phone to the Mountain Police Squad at Annecy on Saturday afternoon, saying his son had fallen and he could no longer see him.
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