The brother of a British-Iraqi man shot dead with three other people in the French Alps last year was arrested in Britain in connection with the murders, French prosecutors said.
Zaid al-Hilli, 54, was detained in Surrey, near London, on suspicion of conspiracy to commit murder in what British police said was a "pre-planned arrest''.
His brother Saad al-Hilli, Saad's wife Iqbal and her mother Suhaila al-Allaf were killed in the family's BMW estate car in a car park at a beauty spot near Lake Annecy on September 5 last year.
A French cyclist, Sylvain Mollier, was also gunned down, although investigators believe he was not a target and was just in the wrong place at the wrong time.
The al-Hillis' two young daughters survived the shooting, one of them by huddling next to her dead mother's body for hours until police found her.
Zaid al-Hilli was taken to a police station after the early-morning arrest in Surrey, where his murdered brother also lived.
Eric Maillaud, the prosecutor for the French city of Annecy, told AFP: "We felt there were enough reasons to take him into custody.
"We need to ask him questions about his schedule, his relationship with his brother and the family inheritance.''
He said searches were being carried out at Zaid al-Hilli's home and the golf and leisure company where he works.
"The family connection remains the preferred line of enquiry, even if no new elements have emerged recently,'' Maillaud said.
Investigators have been looking at whether a possible feud over the estate of the brothers' father could lie at the heart of the case.
In an interview with Britain's ITV News recorded on Friday, Maillaud said documents found at Saad al-Hilli's home suggested the attack may have been timed to prevent him returning to Iraq to lay claim to his father's estate.
The prosecutor said: "There is the hypothesis, involving the family, around the father's inheritance. He didn't have a very big fortune but he had an important fortune in property and we know that both Saad and his brother wanted to recoup it at any price, which created tensions in the family.
"Based on letters we found and conversations he (Saad) had, he feared for his life.
"In these letters he expressed his worry for his life due to his desire to recover his father's fortune and the conflict it caused with his family. That fear was there.''
Prosecutors revealed earlier this month that police were looking into telephone calls to Romania made from Zaid al-Hilli's phone.
They said Romanian authorities had been asked to help establish who he was calling, but stressed on that occasion that the potential Romania link was not being treated as a major new lead.