London (CNN) -- French investigators worked Saturday with British colleagues to go through the English home of a couple mysteriously shot dead while on vacation in eastern France, all in an effort to determine their killer's identity and motive.
The French officers led the search for evidence at the home of two of the attack's four victims, British nationals Saad al-Hilli and his wife Ikbal, in Claygate, Surrey, the county where the family lived.
The al-Hillis are among three people whose bodies were found Wednesday in a car in a secluded parking lot in the foothills of the Alps, near Annecy, French prosecutor Eric Maillaud said.
The couple's two daughters, age 4 and 7, survived the shooting attack, although the elder girl suffered a bullet wound to her shoulder and serious head injuries.
A fourth victim was Sylvain Mollier, a Frenchman who was cycling in the area who was found dead of a gunshot wound to the head in the same parking lot.
Three French investigators arrived in Britain Friday, with a fourth due to join them Saturday.
British officers are carrying out the forensic work on behalf of the French, who are leading the investigation on site, Surrey Police spokesman James Baker told CNN.
Maillaud said Saturday 40 French investigators are involved in the inquiry, as well as a large number of British police and law enforcement in Switzerland and Italy.
"Our cooperation is very good, and the British police are making a due effort, both in human and technical terms," said Col. Marc de Tarle of the French Gendarmerie. "The investigation will be difficult and complex."
Two days after stationing themselves outside, Surrey police officers -- some dressed in white overalls -- set up a tent outside the Al-Hilli family's substantial home in a well-heeled neighborhood Saturday and carried inside materials that appeared to include evidence boxes.
Rob Price, assistant chief constable for Surrey police, said "specialist search teams" from his force were doing "checks" of the al-Hilli family's Claygate residence in support of the French-led investigation. A family liaison officer is assisting relatives of those killed.
"We are also working with our partners, both locally and internationally, to manage the long-term impact of this tragic incident," Price said.
Speaking to reporters earlier Saturday, Maillaud did not offer any more ideas as to why or who might have carried out the brutal attack.
Autopsies confirmed each victim was hit by several bullets, and all four were shot twice in the head, he said.
But he declined to give further details of the autopsy or ballistic tests, saying he did not want to compromise the inquiry. About 25 shots total were fired in the attack, but investigators have not said whether one or more weapons were used.
The bodies will be released to their families for burial as soon as judicial authorities conclude they are no longer necessary for the inquiry, the prosecutor said.
Maillaud said relatives of the two orphaned girls were in France, but he did not disclose who they are or what their relationship is to the children. The youngest, a 4-year-old who hid for hours behind her dead mother's legs before being found, is physically unharmed and being cared for by specialist pediatric medical personnel in France under the eye of police and British consular officials.
All relatives will be questioned as witnesses, he said, including the brother of Saad al-Hilli.
The prosecutor has downplayed reports of conflict between the brothers over an inheritance. The brother went to police voluntarily Thursday after learning from media his relatives had been shot, Maillaud said the following day.
He went back on his own accord the next day to tell police there had been no conflict with his dead brother over money, as had been reported, Maillaud said.
Back in France, investigators have widened to several square kilometers the search area around the shooting scene, near the village of Chevaline in the mountainous Haute-Savoie area.
They are also tracing the family's movements since they arrived in France in late August for a camping holiday, Maillaud said.
One clue may lie in a report by a witness who said he saw a green 4x4 vehicle and a motorbike near the site of the killings.
The slain couple's identities were confirmed to investigators by al-Hilli's 4-year-old daughter, Maillaud said. The girl, identified as Zeena, has not given investigators any more information about who carried out the attack, he said.
Her elder sister, who has been named in media reports as Zainab, is also being protected by police in case of a further threat to her safety. She cannot be interviewed because she's in a medically induced coma as part of her effort to overcome her injuries.
Staff at the local school the 7-year-old girl attended declined to comment on the case.
Their father, Saad al-Hilli, was an Iraqi-born engineer who lived south of London with his wife and two daughters. He was born in 1962 and was a naturalized British citizen. He was completely unknown to French and British counterterrorism and intelligence services at the time of his death, Maillaud said.
Neighbor Jack Saltman, in Claygate, said Saad al-Hilli came from Iraq "many years" ago and both he and his wife spoke perfect English.
"They were a delightful family," Saltman said.
The neighbor said Ikbal al-Hilli was a dentist and the couple's daughters were "absolutely beautiful."
The identity of a third person killed in the car is not yet clear. She was an older woman with a Swedish passport, but her relationship to the others has not been confirmed, Maillaud said.
Swedish Foreign Ministry spokeswoman Linn Duvhammar said the ministry was told by French authorities that the autopsy was completed.
"There is still some technical evidence that they need to work on before they can fully confirm her identity," Duvhammar said.
CNN's Dan Rivers and Per Nyberg contributed to this report.