The home of a British man shot dead in the French Alps along with his wife and two others is to be searched by police.
French prosecutor Eric Maillaud said British and French officers would enter the home of Iraq-born Saad al-Hilli, 50, in Claygate, Surrey.
Brother Zaid al-Hilli has denied a family dispute over money, which is one line of inquiry in the investigation.
Relatives of Mr al-Hilli’s two young daughters, who survived Wednesday’s attack, have arrived in France.
The girls, aged seven and four, are believed to be the only witnesses to the killings, carried out near the popular tourist destination of Lake Annecy, and are under police protection.
Three police officers from France are thought to have arrived in the UK already to investigate the shootings, and a fourth officer is due to arrive later.
The most senior of the four, Colonel Marc de Tarle, who heads the National Gendarmerie Criminal Affairs Bureau, was the first to arrive.
Mr Maillaud said French officers would interview Mr al-Hilli’s brother in the UK as a “witness”.
Surrey Police said they were helping French authorities to carry out a “complex” investigation.
“As part of this, the force is facilitating a visit by French investigators to conduct inquiries in the UK,” a spokesman said.
The BBC understands the family home was originally owned by Saad al-Hilli’s parents and later left to him by his mother in her will.
Police plan to search the house as part of efforts to gather background information about Mr al-Hilli.
Mr Maillaud 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.”
Mr al-Hilli’s wife, Iqbal, and a woman thought to be his 74-year-old mother-in-law, were killed during the attack.
The fourth victim, a cyclist whose body was found near the car after apparently stumbling across the attack, has been named as 45-year-old Sylvain Mollier.
Mr Maillaud said four-year-old daughter Zeena al-Hilli – who spent eight hours hiding in the car with the bodies before being found by officers – 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 – who have not yet been officially named – and hid under her mother’s skirt when the shooting started.
The older sister, Zainab, is in a medically-induced coma in Grenoble University Hospital after being shot once and suffering head injuries.
She was found by a British cyclist who discovered the murder scene and alerted the authorities.
The BBC’s Mark Lowen, in Annecy, said: “Prosecutors hope that if she recovers, she may provide the key to what happened here in this tranquil corner of France, and who is to blame.”
Relatives of the girls arrived in France on Friday night, accompanied by a British social worker, and were due to meet Zeena later, Mr Maillaud said.
“I don’t know when they will be able to see the little girl. We have to be sure it can be done without problems,” he told AFP news agency.
During a press conference on Friday, the prosecutor confirmed that each of the victims had at least one bullet in the head, and about 25 shots were fired in total – more than originally thought.
This has led investigators to believe that more than one gun was used in the killings.
They are looking into reports of a green or dark-coloured four-wheel drive vehicle and a motorcycle, apparently seen by the cyclist who discovered the murder scene. But Mr Maillaud pointed out that such vehicles were common during the tourist season in a mountainous region.