France President Francois Hollande has said everything will be done to find the killer or killers who shot dead four people in a remote Alpine wood close to the Swiss border.
Three bodies were found inside a British-registered car. The body of a fourth victim, a passing cyclist, was found close-by.
Two children who had been in the car but survived the attack continue to be treated in a hospital in the nearby city of Grenoble.
The driver of the car, an elderly female passenger and the cyclist were all shot in the middle of the head.
Public prosecutor Eric Maillaud described the attack 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.
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, 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.
Mr Maillaud praised the cyclist at today’s press conference.
Police said the motive for the attack remains 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.
