The family were on a camping holiday when the attack took place
The shooting of a British family in France "was like something out of a Hollywood movie", a British man who was first on the scene has said.
Brett Martin helped seven-year-old Zainab al-Hilli after the attack which claimed the lives of her parents, grandmother and a local man.
He told the BBC he made the difficult decision to leave her bleeding on the ground while he went to seek help.
Meanwhile, a French prosecutor said the motives "have their origins" in the UK.
Eric Maillaud was speaking outside Surrey police station after arriving with the French judge leading the inquiry.
French and British police have been searching the Surrey home of Saad al-Hilli, 50, was shot with his wife, mother-in-law, and a passing cyclist in Chevaline, close to the tourist destination of Lake Annecy.
Zainab and her four-year-old sister Zeena both survived the attack.
Mr Martin, a former member of the RAF from Sussex, said he initially believed there had been a car accident after coming across the scene while cycling in the forest area.Police protection
He saw three bodies in the car, and once he realised a crime had taken place, he was worried the gunman might still be roaming around.
He told the BBC the car's engine was running and its wheels were spinning when he arrived.
He moved Zainab, who had collapsed on the ground, away from the car in case it began moving.
Mr Martin added that he travelled back to the location of the shooting under armed police protection to give further details to French officers.
French police have spoken briefly to Zainab, who remains in hospital in France after being shot in the shoulder and hit around the head. But they are still waiting to question her fully about what she can remember of the attack.
Zeena has returned to the UK and is under the care of social services.
She lay undiscovered in the car for eight hours after her parents, her 74-year-old grandmother, and a local cyclist, Sylvain Mollier, 45, all died.
They were staying in a caravan at the Le Solitaire du Lac campsite in nearby Annecy.