For as long as eight hours, the small girl hid, unnoticed, in a car with the bodies of two British women and a man shot dead in foothills of the Alps in eastern France.
Probably paralyzed by fear, the 4-year-old remained among the corpses as investigators waited for crime scene technicians to arrive so the car could be opened.
She is now under police protection, as is an older girl, thought to be 7 or 8 years old, who was found injured near the car. She suffered a fractured skull and an apparent bullet wound to the shoulder.
Prosecutor Eric Maillaud told reporters Thursday that investigators failed to notice the younger girl because she didn't move for hours as they waited for the forensic experts.
She was hidden under baggage and the legs of one of the dead women in the car's back seat. A thermal imaging camera failed to pick up her presence, Maillaud said.
The child was "probably terrorized, completely concealed, completely immobile amid the corpses," he said.
Authorities have not identified the three Britons found dead on Wednesday in the car near Lake Annecy, a popular tourist spot in the Haute-Savoie area of eastern France. A fourth victim, a French cyclist found shot in the head near the car, has been named as Sylvain Mollier.
British Prime Minister David Cameron said he and French President Francois Hollande discussed the "terrible killings of ... the British family" during a meeting Thursday in London. He promised British authorities will cooperate "very closely with the French authorities" to "help those poor children" and "get to the bottom of what happened in this clearly very tragic and awful case."
"Both a French and a British family have been impacted by this terrible event, and we will do our utmost to identify the perpetrators," added Hollande in a joint press conference after the two leaders' meeting.
Maillaud, the French prosecutor, has called the shootings an act of "extreme savagery" but has declined to speculate on a motive.
"The scene is dramatic, it's unusual -- it goes well beyond TV fiction," he said.
Multiple bullet casings had surrounded the BMW when authorities came upon it, with bullet holes peppering its windows but none in the body of the car, according to the prosecutor.
The driver and an older woman found dead in the car had been shot in the head, Maillaud said, "and it was obvious that (whoever) did this wanted to kill." Autopsies are scheduled Friday and should reveal how the other passenger, a younger woman, died.
The car was registered to a man with an Iraqi passport who was a naturalized British citizen and had lived in Britain since at least 2002, he said. The passport was used when the visitors checked in to the campsite where they were staying, Maillaud said, but police can't yet confirm whether it belonged to the man who was found dead in the car.
The man named in the passport was born in 1962, the French prosecutor said. A Swedish passport found by investigators may belong to the older woman, he added. The victims are believed to be British nationals but may have held dual citizenship.
Swedish Foreign Ministry spokeswoman Catarina Axelsson described "the French information about the Swedish national (as) reliable," adding that authorities there were awaiting a "final confirmation of the identity."
DNA tests are being carried out to formally identify the victims.
French and British news reports have identified one victim as Saad al-Hilli, an Iraqi-born engineer who lived in Claygate in Surrey, south of London, with his wife and two daughters.
While CNN couldn't independently confirm the identities of the victims, Surrey Police have said they are helping police in Annecy with their investigation into the killings, though they haven't given further details.
Officers from Surrey Police were stationed Thursday outside the house identified by neighbors as that of Saad al-Hilli.