Police threw up a cordon around the home of Saad and Ikbal al-Hilli, who were killed along with Ikbal al-Hilli's 74-year-old mother in the French Alps.
Officers cleared residents from houses around the al-Hilli home in Claygate, Surrey, a wealthy town outside of London, police in Surrey County said.
A French cyclist was killed alongside the British-Iraqi family members Wednesday on the outskirts of Chevaline, an Alpine village popular with outdoor enthusiasts.
Each victim was found with two gunshot wounds to the head.
The couple's 7-year-old daughter was badly beaten and shot. Their 4-year-old daughter hid for hours behind her dead mother's legs.
The attack was, in the words of one French prosecutor, an "unheard-of savagery."
Days after the incident, French and British authorities are still trying to piece together the puzzle.
Was it a robbery gone bad? A family feud over inheritance? A case of mistaken identity? Or, maybe, one of being at the wrong place at the wrong time?
Theories have abounded since investigators found the bodies.
The fourth victim was Sylvain Mollier, a Frenchman who was cycling in the area.
About 25 shots were fired, police said.
Authorities have been tight-lipped about possible perpetrators and motives, even as the investigation stretched Sunday from France to Britain and they asked Italian and Swiss officials for help.
The answers may rest in part with the survivors of the shooting: the couple's two daughters.
The 4-year-old, identified in media reports as Zeena, has not given investigators any information about who carried out the attack.
She is physically unharmed and will be returned to Britain, French prosecutors said Sunday. She has been under the care of French doctors, watched over by police and British consular officials.
Her elder sister, who has been named in media reports as Zainab, is also being protected by police in case of a further threat to her safety. She suffered head wounds and a gunshot wound to the shoulder, but came out of a medically induced coma Sunday, the French prosecutor's office said.
One clue may lie in a report by a cyclist who said he saw a green 4x4 vehicle and a motorbike near the site of the killings.
That cyclist, identified as a former member of the Royal Air Force in media reports, discovered the bodies at the rest stop. The engine of the al-Hillis' car was still running, according to authorities.
The al-Hilli family arrived in France in late August for a camping holiday, French prosecutor Eric Maillaud said.
Al-Hilli was an Iraqi-born engineer who lived south of London with his wife and two daughters.
He was born in 1962 and was a naturalized British citizen. He worked at Surrey Satellite Technology, a high-tech company owned by EADS, an aerospace corporation that builds satellites.
Neighbor Jack Saltman, in al-Hilli's well-heeled Surrey County community of Claygate, said al-Hilli came from Iraq "many years" ago.
Police plan to question al-Hilli family members, including his brother, Maillaud said.
The prosecutor downplayed reports of a conflict between the brothers over an inheritance.
The unnamed brother went to police voluntarily after he learned of the deaths, Maillaud said. He returned the next day on his own accord to tell police there had been no conflict with al-Hilli over money.
Investigators have not ruled out the possibility that the al-Hillis were killed in a robbery at the rest stop, and that the cyclist may have been killed after stumbling upon the robbery.
The bodies will be released to their families for burial as soon as judicial authorities conclude they are no longer necessary for the inquiry, the prosecutor said.