Detectives investigating a multiple murder in the French Alps last September, including a British couple, have arrested the brother of one of the victims.
French and British police have been working together to find the killer of Saad Al-Hilli, his wife, his mother-in-law and a French cyclist.
ITV News Europe Correspondent Emma Murphy reports.
The brother of Saad Al-Hilli is being questioned by detectives investigating the killing in the Alps of Saad, his wife Ikbal, mother-in-law Suhalia al-Allaf and local man Sylvain Mollier.
Mr Al-Hilli's brother Zaid Al-Hilli, who is in his 50s and lives in Surrey, has previously denied any feud with his sibling over an inheritance. He was arrested by police at an address in Chessington.
Around 100 police officers in Britain and France are investigating the killing of the family on September 5 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.
A man held by police in connection with the deaths of the Al-Hilli family in the French Alps last year has been arrested on suspicion of conspiracy to commit murder.
Surrey Police said the suspect, who they did not name was held at an address in Chessington at around 7.30am on Sunday "and is currently in police custody where he will be interviewed".
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 injures after being shot and beaten.
Surrey police said today's arrest was pre-planned and the result of on-going inquiries.
A 54-year-old man was arrested in Surrey today in connection with the murders of Saad and Ikbal Al-Hilli, Mrs Al-Hilli's mother Suhaila al-Allaf, and cyclist Sylvain Mollier, who were all shot dead near Annecy, in the French Alps in September last year, Surrey Police said.
Zainab al-Hilli's health is understood to have improved in recent days, allowing her to travel back to the UK.
Zainab left France this morning at around 8am. Police from Britain accompanied the little girl.
French prosecutors have told ITV News that the search for a black Mitsubishi Pajero with British number plates is "not connected at all" with the French Alps shootings.
Sky News is reporting that French police investigating the fatal shooting of a British family in the Alps are looking for a black Mitsubishi Pajero with British number plates.
Eyewitnesses told French police that the driver of the car looked "rather nervous", according to the news channel.
Surrey Police has told ITV News that Zainab al-Hilli, the seven-year-old British girl who was wounded in the French Alps shootings in which her parents were killed "is returning to the UK."
She has been in hospital being treated for a fractured skull and gunshot wounds suffered in the massacre.