A French judge and prosecutor are to travel to the UK, as part of their investigation into the murder of three Britons in the Alps.
They will join a small team of French investigators already in the country, to find out what led to the deaths of Surrey residents Saad al-Hilli, his wife Iqbal and his mother-in-law.
They were gunned down in a car park near Lake Annecy a week ago, together with a French cyclist.
Mr and Mrs al-Hilli's daughters Zainab, 7, and Zeena, 4, managed to survive the attack, although Zainab was so badly hurt that doctors placed her in a medically induced coma for several days.
She is now seen as one of the key witnesses. Police spoke briefly with her after she regained consciousness on Sunday and are waiting for approval from medics before they can question her further.
Zeena has now returned to Britain, after escaping physical injury by hiding under her mother's skirt during the attack.
A witness who came upon the aftermath of the Alps massacre likened the carnage to a horrific film scene.
The man, named only as Philippe D, told Le Parisien newspaper how he came across the dramatic scene after setting out with two female friends to go walking:
"It was like in a film," he said. "One of those television series where everything begins with a murder, only this time we were the actors and we did not have the remote control to change the channel."
He added: "I approached the car. I did not touch anything but I saw that there was nothing that could be done. There was no sign of life."
He spoke out as it emerged the al-Hilli family had moved from one campsite to another two days before they were gunned down.
Sources said the victims were likely to have been blasted with the same gun, fuelling speculation they were targeted by a contract killer. But Annecy's public prosecutor Eric Maillaud has denied confirming this detail.
Officers are now examining all aspects of Mr al-Hilli's life to find a motive for the murders, looking at his personal and professional links which include work for a satellite technology company in Surrey.
Detectives in Surrey have been searching Mr al-Hilli's home, in Claygate, while French police are examining two mobile phones found in his car.
Meanwhile, three more people were found in a car following a shooting on the French island of Corsica yesterday, although there was no immediate evidence of any link between the two attacks.