CHEVALINE, France (AP) — French investigators struggled Thursday to explain how a 4-year-old girl could go undetected for eight hours in a car full of corpses in the French Alps.

They also acknowledged that they still don't know why a family of British vacationers were slain in a BMW on a remote mountain road near the French village of Chevaline.

The attacker or attackers violently beat and shot the girl's sister, who is about 7 years old, Prosecutor Eric Maillaud told reporters in nearby Annecy. The older girl was found near the car and hospitalized. She will be operated on but her life is out of danger, he said.

French authorities were not releasing the identities of the victims, but said the car was registered to a man with a British passport, born in Baghdad in 1962. A Swedish passport and Iraqi passport were also found at the scene, Maillaud said.

Four people were found dead on the remote road: one adult man in the driver's seat of the British-registered BMW; two women in the back seat, one older than the other; and a French male cyclist who appeared to have nothing to do with the family.

The bodies were found Wednesday by a British former air force officer who was cycling by, the prosecutor said.

A number of rescuers — firefighters, medical workers, police — apparently eyed the crime scene after it was reported about 4 p.m. on Wednesday. Local officials then waited for special investigators to arrive, police said.

The 4-year-old girl was only found after midnight, but was doing fine physically, Maillaud said. She described hearing cries and asked investigators where her family was. She was taken into police care and will be questioned later, he said.

The prosecutor said they were looking at all possible motives and are protecting the girls in case the killers are still on the loose and want to "get rid of" witnesses to the killings.

Three of the four victims were shot in the head, and the fourth victim remains in the car pending further investigation, Maillaud said.

He said the bodies will be autopsied Friday.

"We strictly don't know why these people were killed," Maillaud said.