The brother of a British engineer shot dead with his wife and mother-in-law in the French Alps last year was arrested in Surrey today on suspicion of conspiracy to commit murder.
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.
Police in Surrey said the 54-year-old suspect, who they did not name, was detained at an address in Chessington at around 7.30am "and is currently in police custody where he will be interviewed".
Mr Al-Hilli's brother Zaid Hilli, who is in his 50s and lives in Surrey, has previously denied any feud with his sibling over an inheritance.
Surrey Police set up a joint investigation team to work with French police on solving the brutal 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 injuries after being shot and beaten.
Zainab was placed in a medically induced coma following the attack. She left hospital in France and was able to return to the UK.
Surrey police said the arrest was planned and the result of ongoing 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.
In September they said 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 perhaps understand a real motive, the real reasons for these killings.
"Perhaps if we can understand why they were killed we can work out who killed them, but at the moment there are many questions.
"I think the investigation will take a very, very long time, unless we discover something that will suddenly enable us to understand everything."
Detectives investigating the massacre appealed in April for help to trace a vehicle seen close to the scene just before the deaths.
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, at about 3.20pm on 5 September.