The chief prosecutor in the area of the French Alps where three Britons were gunned down said that the causes and origins of their deaths lay in this country.
Annecy's chief prosecutor Eric Maillaud made the comments as he travelled to the UK as investigators continue to probe the mystery.
Mr Maillaud and examining magistrate Michel Mollin, another senior member of the inquiry team, met officers from Surrey Police at Woking police station to discuss the case.
Engineer Saad al-Hilli, 50, and his dentist wife Iqbal, 47, were brutally murdered along with her mother in a remote spot close to Lake Annecy a week ago.
The couple's four-year-old daughter Zeena lay undiscovered under her mother's corpse for eight hours afterwards, while her seven-year-old sister Zainab was found with serious injures after being shot and beaten.
On Wednesday their families released a statement in which they said they were "heartbroken" by the deaths.
Mr Maillaud told reporters outside Woking police station it was "without any doubt that the reasons and causes have their origins in this country". He added: "It is only by being together that we will find the murderers." He thanked English authorities for their help with the inquiry.
Assistant Chief Constable Rob Price, who is leading the Surrey Police team, met the dignitaries.
He said: "As I stated previously, this is a French-led investigation. Surrey Police and other agencies within Surrey will do all we can to support this investigation. Inevitably, when you have different jurisdictions, there will be complexities and challenges.
"What I am please to report is that we have overcome these challenges and we are all determined to overcome future challenges. We are all operating with a determination and a focus and always have in mind the victims and all those who have been affected by this tragic incident."