Prosecutors have said the four people gunned down in the French Alps had been shot twice in the head.
Fifty-year-old Saad al-Hilli was killed in his car alongside his dentist wife and an older Swedish woman. Cyclist Sylvain Mollier, who apparently stumbled across the attack in Chevaline, was also killed.
The couple's four-year-old daughter Zeena lay undiscovered under her mother's corpse for eight hours after the murders. Her seven-year-old sister Zainab remains in a coma after being shot and beaten.
French detectives have been in Claygate, Surrey, searching al-Hilli's home. Forensic officers are scouring rooms for clues to why the murders took place.
Police are investigating whether there had been a family feud over money but Mr al-Hilli's brother has already denied the speculation.
The chief prosecutor has said that all immediate family members will be questioned. A number of family members are to be allowed to visit Zeena al-Hilli but under police supervision.