British police search teams have arrived at the home of the Alps murder victims and are set to be joined by French officers in the hunt for clues.
They placed a tent in the driveway of Saad al Hilli's house in Claygate, Surrey, ahead of an expected forensic examination of the property.
Several French detectives have gone to the UK as part of their investigation, as they look for a motive for the murders of four people.
Police from the Haute-Savoie region will work with British officers following the shootings on Wednesday.
French investigators also plan to interview Mr al Hilli's brother - who has approached UK police to deny any feud between the siblings over money.
Iraqi-born Saad al Hilli, 50, was shot dead in his BMW alongside his dentist wife Iqbal while on holiday close to Lake Annecy.
An older Swedish woman who was travelling in the car was also killed, along with Sylvain Mollier, 45, a French cyclist who apparently stumbled across the attack in Chevaline, near the borders of Italy and Switzerland.
Around 40 French investigators are working around the clock on the case. Police in Switzerland and Italy are also helping in the hunt for those behind the shooting.
The couple's four-year-old daughter Zeena lay undiscovered under her mother's corpse for eight hours after the murders, while her seven-year-old sister Zainab remains in a medically-induced coma after being shot and beaten.
Two close family members of the young girls have arrived in the region with a British social worker. They will be allowed to visit the children, but only under the supervision of detectives.
The two orphans have been in the care of British consular staff and nurses.
French prosecutor Eric Maillaud said Zeena has been looked after by psychiatric teams and had spoken about what he described as the "terror" of what happened, but did not see anything because she was hiding.
Zainab is not yet well enough to be interviewed, but it is hoped she will be able to provide vital details of the attackers.
Mr Maillaud said police plan to look at all aspects of Mr al Hilli's life as they try to establish why the family may have been targeted.
He said: "Up until now the police in Britain were guaranteeing the safety of the house, but now it's a Franco-British inquiry that is starting and we can now enter the house of Mr al Hilli.
"His life, his job - I heard that he was the owner of three companies - all of that is the sort of thing that we have to find out about in England."
A Scotland Yard spokesman said they could not comment, but it is understood there is no link between the deaths and any national security issues.
Surrey Police said it was assisting the French authorities as they carry out a "complex" investigation.