Saad and Ikbal al-Hilli, Mrs al-Hilli’s mother, Suhaila al-Allaf, and local man Sylvain Mollier were murdered on a remote mountain road near Annecy.
The 54-year-old suspect, Zaid al-Hilli, was detained at an address in Chessington, Surrey, at around 7:30am yesterday and was in police custody, where he was to be interviewed.
The man has previously denied any feud with his sibling over an inheritance.
Six members of a police search team left the property in Chessington at about 2:25pm yesterday. They carried items including a plastic box, a ladder and two bags of evidence to a police van.
Neighbours said they had not heard or seen any sign of yesterday’s arrest.
One man who lives in the flat below al-Hilli described his neighbour as being unassuming and a “lovely man”. Philip Davies said he had seen police at the address “on various occasions”.
He said of al-Hilli: “Nobody really has had anything to do with him. He’s been living here about a year or 18 months.”
Surrey Police set up a joint investigation team to work with French police on solving the murders. The al-Hillis’ four-year-old daughter, Zeena, lay undiscovered under her mother’s corpse in their BMW for eight hours after the shooting, while her seven-year-old sister Zainab was found with serious injures after being shot and beaten.
Surrey Police said yesterday’s arrest was pre-planned and had been the result of on-going inquiries.
Around 100 police officers in Britain and France are investigating the killing of the family on 5 September last year as they drove through a remote area close to Lake Annecy.
French investigators came to the UK and searched the al-Hilli family home in Claygate, Surrey, in the wake of the deaths.
They said in September that they were investigating three lines of inquiry, focusing on Mr al-Hilli’s work, his family and links to his native Iraq.
Annecy’s chief prosecutor, Eric Maillaud, said last year that there would be no “quick solution” in finding the killers.
Mr Maillaud said: “We’re investigating everything, but it all takes a lot of time, trying to piece together the lives of all the people who have died, trying to understand a real motive, the real reasons for these killings.
“I think the investigation will take a very, very long time, unless we discover something that will suddenly enable us to understand everything.”
Detectives appealed in April for help to trace a vehicle seen close to the scene just before the murders.
Officers said they were keen to speak to the owner of a right-hand drive 4x4 vehicle, possibly a grey, black or dark-coloured BMW X5, which was being driven on the Combre d’Ire Road, Chevaline.
Zainab, who was shot in the shoulder and badly beaten, was placed in a medically induced coma following the attack. She left hospital in France and was able to return to the UK.
Brett Martin, a cyclist, was the first witness to arrive at the scene where Mr al-Hilli, 50, his 47-year-old dentist wife and her elderly mother died.
He described how he saw Zainab “stumbling” around, bleeding and “moaning” near the car.
The former RAF pilot likened the carnage to a set from TV crime series CSI: Miami.