British and French police are due to search the home outside London of an Iraqi-born man shot dead with two members of his family while on holiday in France.
French investigators will also interview the brother of Saad al-Hilli, who has denied reports of a family dispute over money.
Mr al-Hilli's two daughters, four and seven, remain in hospital in France after the attack.
The BBC is reporting that at least four French police officers will work with their British colleagues, looking into the al-Hilli's family background for any clues to the killings.
French investigators are also continuing their forensic work. They found 25 bullet casings at the scene of the crime.
Post mortems show the three members of the al-Hilli family and a French cyclist, who was also killed, had multiple gunshot wounds.
Each had at least one bullet wound to the head.
Speaking at a media conference on Friday, Prosecutor Eric Maillaud said the brother of Saad al-Hilli, 50, went to the police after he heard media reports of the deaths.
He contacted police first to ask about Mr al-Hilli's condition, and then, on Friday, to deny reports of a dispute with his brother.
Mr Maillaud said that four-year-old Zeena al-Hilli had identified her family, and described the "fury" and "terror" of the attack to French police.
The child said she was between her mother and the older woman in the car - who has not yet been officially named - and hid under her skirts when she the shootings began.