French police have enlisted the help of Italy and Switzerland in their investigation into the killing of a British-Iraqi family who were on holiday in the Alps.
A passing cyclist was also killed.
In Britain, four French officers are working with local detectives in the house outside London of the murdered man, Saad al-Hilli.
Mr al-Hilli, his wife, a woman thought to be his mother-in-law and a local cyclist were shot twice in the head reports the BBC.
The couple's two daughters survived the attacks and are in hospital in France.
Senior French police officer Colonel Marc de Tarle said the investigation into the shootings is likely to be "long and complex".
Colonel Marc de Tarle was speaking outside Surrey Police HQ after his officers searched Saad al-Hilli's home nearby with a British forensics team.
Surrey Police assistant chief constable Rob Price said: "Surrey police will do all we can in support of the investigation on behalf of our French colleagues.
"Throughout that support, I want to place the emphasis on the victims of this tragic incident and Surrey police, again with our French colleagues, are ensuring that all those who need support will get the support."