Public prosecutor Eric Maillaud described the attack, which left the driver, two women and a passing cyclist dead, as an act of "gross savagery".
A four-year-old girl was found alive underneath the bodies of her family around eight hours after the massacre.
And a seven-year-old girl, thought to be her sister, was found violently beaten and is in a coma, fighting for her life.
The four-year-old lay undiscovered as police waited for forensic experts to arrive at the scene from Paris. She was found "terrorised, motionless, in the midst of the bodies" after fellow campers at a nearby site told officers the family had two children, Mr Maillaud said.
He defended the delay in finding her as he revealed she was receiving psychiatric treatment.
A man and two women - believed to be the girls' mother, father and an older relative - were in the British-registered BMW, which was riddled with bullets.
The man was named by French media as Saad al-Hilli, from Claygate, near Esher in Surrey.
Mr al-Hilli was behind the wheel at the time of the attack.
Mr Maillaud refused to confirm his name but revealed that he was 50 years old, was originally from Iraq and held British citizenship.
A British cyclist, a former member of the RAF, who stumbled upon the shocking scene and spotted the seven-year-old lying in the road, placed her in the recovery position.
Speaking at a press conference today he was praised by Mr Maillaud, who said the four-year-old was in a traumatised condition.
"Can you imagine the state of the young girl, four years old, has spent eight hours under a body that was not moving..."
Police said the motive for the attack remained a mystery but revealed there were signs of a vehicle braking at the scene.
Mr Maillaud also revealed that Swedish and Iraqi passports had been recovered along with the driver's British passport.
Mr Maillaud said the British cyclist was overtaken earlier by the French bicycle-rider found shot at the scene.
He said the British man told them he came across the "powerful" BMW estate with the engine still running.
"He also saw coming towards the car a young girl who collapsed before him," he said.
"He quickly came to her and put her in a side position and called the police and ambulance service and everyone came to the scene."
He said the girl had been repeatedly hit "extremely violently".
The British cyclist then went the vehicle and found the French victim by its side. He broke a window and turned the car engine off and found the other three bodies in the car.
"We are talking about particularly horrific and alarming deaths," he added.
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