(CNN) -- French police investigating the mysterious shooting deaths of four people in the foothills of the Alps hope autopsies to be performed Friday will provide new clues to the identity of the killer.
Annecy prosecutor Eric Maillaud has opened a judicial investigation for murder and attempted murder in the case, Joelle Robert of the Annecy prosecutor's office told CNN.
She declined to confirm media reports that Maillaud has said he is investigating a potential family feud, among other leads.
A man and two women, all thought to be British nationals, were found dead in a car, two of them shot in the head, on Wednesday in a parking lot in the Haute-Savoie area of eastern France, near Lake Annecy.
The fourth victim, French cyclist Sylvain Mollier, was found with a gunshot to the head in the same parking lot off a forested road.
Police hope to glean some information from the two survivors of the grisly incident, a 4-year-old girl who was found hidden among the corpses in the car and a girl thought to be age 7 or 8 who was injured.
Both girls have been kept under police protection since they were discovered in case of possible threats to their safety.
The younger girl has so far spoken only to say she heard noise and was scared. The older girl, who suffered head injuries and a bullet wound, was placed in a medically induced coma, Maillaud said Thursday. She faces further surgery.
Authorities have not identified the three Britons found dead. DNA tests are being carried out formally to identify them.
Maillaud said autopsies would be carried out Friday. The results are expected later Friday or Saturday morning, CNN affiliate BFM-TV reported.
The grisly case has captured headlines in both countries, sparking wide speculation about possible motives for the killings, from a family feud to a drug deal gone wrong.
French authorities have said they are ruling nothing out in their investigation into what Maillaud described as an act of "extreme savagery."
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 al-Hilli.
Neighbor Jack Saltman, whose home backs onto the family's garden, said al-Hilli and his wife had come from Iraq "many years" ago and both spoke "perfect English."
"They were a delightful family," Saltman said.
The neighbor said al-Hilli's wife, Iqbal, was a dentist and that the two daughters were "absolutely beautiful." He had been asked to keep an eye on the house while his neighbors were away in France, Saltman said.
An accountant for Saad al-Hilli, Julian Stedman, said the engineer had at least one business registered locally.
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 news conference after the two leaders' meeting.
The gory scene was discovered Wednesday afternoon by a passing cyclist, a retired British serviceman who has not been named.
After helping the injured girl, who was near the car, he alerted emergency services.
The younger girl had spent nearly eight hours inside the car with the three corpses before French police, who were waiting for forensic experts to arrive before opening the vehicle, were made aware of her likely presence and discovered her hiding under the legs of one of the women.
Multiple bullet casings surrounded the BMW when authorities came upon it, with bullet holes piercing its windows but none in the body of the car, according to Maillaud.
The driver and an older woman found dead in the car had been shot in the head, the prosecutor said Thursday, "and it was obvious that (whoever) did this wanted to kill."
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."