FOR at least eight hours, the four-year-old girl lay motionless, frightened and alone, hiding under her dead mother's skirt on the floor of a BMW after a killing spree in the French Alps.

As the hours ticked by, police peered through the car's windows at the lifeless bodies of her parents and grandmother.

Only after police forensic experts finally arrived shortly before midnight from Paris, peeling back the blood-stained layers of the grisly crime scene, did the girl crawl out apparently unharmed - asking, with a smile and hugs, where her family was.

The motive for the slayings of the family and a French cyclist whose body was found nearby remained unclear today, a day after the bodies were found in a wooded area up a mountain road from the village of Chevaline, near bucolic Annecy Lake.

Earlier, rescuers had whisked the child's wounded seven-year-old sister to a hospital as helicopters scanned the area. All the while, immobilised by fear, the girl had remained huddled alone, steps away from investigators.

Never did they think anyone else was still alive.

media_camera Gendarmes and investigators stand at the camp site where the slain British family were holidaying in Saint Jorioz, near Annecy. Picture: AP/Lionel Cironneau

"The girl was found totally immobile behind the front passenger-side seat, under the legs - under the skirt - of one of the women, hidden behind a large travel bag, totally invisible and silent, which explains why no one saw her before," French state prosecutor Eric Maillaud told a news conference in Annecy.

The case took on international ramifications, with links that tied the slain family to Britain, Iraq and Sweden.

The murders first came to light when a British cyclist telephoned authorities after passing the body of the slain Frenchman. The Briton - a former Royal Air Force pilot - then saw the badly wounded older girl lunging toward him. The car's motor still running, he laid the child on the ground to give her first aid that French authorities said saved her life.

The go-ahead to open up the car and start unraveling the mystery was given only after police forensic experts flew in - hours after the British cyclist's call. Authorities insisted they didn't want to compromise the crime scene.

Maillaud said three of the victims - the man, the elderly woman and the French cyclist - had gunshot wounds to the head, while the cause of death of the younger woman was still under investigation.

media_camera Gendarmes escort the car involved in the shooting from the scene. Picture: AAP/EPA

"We strictly don't know why these people were killed," Maillaud said, adding that about 15 bullet casings were found around the car. "What is certain is that someone wanted to kill."

He said authorities were searching for suspects and had not ruled out anything, including the possibilities that the attack was intended to settle a score or simply that the family was "in the wrong place at the wrong time."

The two children were put under police protection, and the prosecutor warned that the killer might try to "get rid of" witnesses to a "scene of immense savagery."

Maillaud said the BMW station wagon was registered to a British man born in Baghdad in 1962. The man, whom he did not identify, had lived in Britain since at least 2002, and his family had been in France since August, camping at Annecy Lake since Monday.

The Sipa news agency identified the driver as Saad al Hilli, a resident of a London suburb. Public records identified him as a mechanical engineer and his LinkedIn page described him as an aerospace consultant.

media_camera Journalists gather at a house believed to be owned by a British family murdered in the shooting. Picture: Peter Macdiarmid/Getty Images

Sky News, citing neighbors in the British village of Claygate, identified al Hilli's wife as Iqbal, the 7-year-old as Zehab and the 4-year-old as Zeinab. Maillaud cautioned that it was too early to confirm any identities, pending results of DNA and fingerprint tests.

Sweden confirmed one of the victims was Swedish. French authorities found a Swedish passport that apparently belonged to the older woman, who was born in 1938, as well as an Iraqi passport.

The French cyclist was identified as Sylvain Mollier, 45, from nearby Grenoble who police believe had no connection to the British family. His wife had called police after Mollier failed to return from a ride.

The al Hillis' camper was covered in red police tape today, and the case sparked fear among other vacationers. A British retiree at a neighboring campsite said he was considering leaving after hearing what happened to the family.

The slain victims and the wounded girl were found just before 4pm Wednesday by the British cyclist, who has a house in the region. Questioned about the eight-hour delay in finding the second child, Maillaud said police were trying to keep the crime scene intact to allow forensics and other experts to arrive.

media_camera Annecy prosecutor Eric Maillaud talks to journalists near the scene near Annecy Lake in eastern France. Picture: AFP/ Jean-Pierre Clatot

"When dealing with such a big crime scene the main thing ... is to make sure this investigation is in no way compromised," he said.

Police told reporters helicopters using thermal scanners did not detect the surviving child in the car, perhaps because her body heat was concealed by the women's bodies she was hiding under.

It was only after fellow campers alerted investigators that the missing family had two daughters that police rushed back to the crime scene.

When investigators opened the car door, the four-year-old emerged, smiling, and reaching out, Maillaud said. Speaking English, she said she heard cries, but couldn't describe what had happened. She was unharmed physically and will be questioned later, he said.

"She quickly asked where her family was," he said. "We are hoping for more information from her sister that will help the investigators to move forward."

David Wilson, a criminologist at Birmingham City University in England, said it was understandable that French police wanted to preserve the integrity of the crime scene, but questioned why it took so long before the child was found.

"Every crime scene these days is treated with almost religious significance," said Wilson. "Police officers - whether we're talking about France or the United States or elsewhere - have this drilled into them: `You don't contaminate the crime scene.' "

The complex nature of the scene notwithstanding, he said police blundered.

"Even in those kinds of crime scenes, you have to check for life," he said. "Eight hours is an incredible length of time for the little girl not to have been found."

Later Thursday, hearses under police escort and a tow truck hauling the BMW left the area. Autopsies were planned for Friday.

media_camera Journalists wait in front of Gendarmes, who block access to a site where people were shot to death near Chevaline, French Alps. Picture: Laurent Cipriani

French President Francois Hollande, who met with Britain's Prime Minister David Cameron in London, promised that French authorities "will do our utmost to find the perpetrators."

Cameron said consular staff were assisting the girls and "are working very hard ... to find out what happened in this very tragic case. Obviously the faster we can get to the bottom of what happened, the better."

Britain's ambassador to France, Peter Ricketts, said British diplomats were trying to contact other family members but so far none had been located. He declined to say whether the father's Iraqi ties may have been a factor in the killings.

"All I can say is that we are doing everything you would imagine that we would do for a small girl who has been deeply traumatized in a foreign country where she doesn't speak the language. We are doing everything we can for her," he said.

An accountant at al Hilli's computer design company, Shtech, said he had spoken with al Hilli the day before the family left for France and that his mother-in-law was probably the older woman in the car, according to Britain's Press Association.

Julian Stedman described al Hilli as a "hardworking family man who loved his children."

Mae Faisal El-wailly, a childhood friend of al Hilli's in Iraq, described him as an outgoing child from a well-off family.

"As a child he was very active, very happy," said El-wailly, who now lives on Phoenix and said she had reconnected with al Hilli through social media. "He liked photography, traveling, and cycling ... He seemed very happy."