French police have arrested a suspect in the case of the murders of the Iraqi-born British engineer Saad al-Hilli and three others in the French Alps in September 2012.
The breakthrough came after police issued an efit of a man wearing a motorcycle helmet.
BFM-TV, which broke the story, reported that the 48-year-old man, who can be questioned by police for 48 hours, is from the Alpine area where the murder took place.
Eric Maillaud, the Annecy prosecutor, said a suspect matching the efit had been "actively sought by police".
Al-Hilli, who was staying in Annecy with his family, was shot dead on 5 September 2012 after parking his BMW in a layby on a wooded road near Chevaline. His wife and mother were also shot dead.
One of his daughters was wounded but the other miraculously survived by hiding under her mother's skirt. The body of a passing French cyclist, Sylvain Mollier, was found nearby.
Police were puzzled by the weapon used by the killer: an antique 7.65mm Luger P06 handgun, issued to the Swiss army and police in the 1920s and 1930s.
French police made the arrest following witness accounts and descriptions of a man seen near the scene of the killings. The witnesses came forward after French authorities released an artist's impression of a motorcyclist who was seen in the area between 3.15pm and 3.40pm shortly before the four murders.
The prosecutor suggested that there may be other arrests. "This arrest, which may not be the only one, is the fruit of witness statements taken notably after the release on 4 November 2013 of the e-photo of a motorcyclist seen near the scene of the crime, and actively sought by investigators," Maillaud said.
However, French media reports said police remained cautious as to the possible involvement of the person taken into custody on Tuesday. They said he may be able to shed light on the murders without being a direct suspect in the killing himself.
Al-Hilli's brother Zaid was arrested by British police but released from bail last month when Surrey police deemed there was not enough evidence to charge him with a crime.
The 54-year-old, from Chessington, Surrey, said he was relieved, but French investigators said they still had "many questions" to ask him.
The al-Hilli brothers were alleged to have been locked in an inheritance dispute centred on the £825,000 home in Claygate, Surrey, where Saad and his family lived after their mother died from a heart attack in 2003.
Zaid, who inherited half the property, claimed that in 2011 his brother began to demand his share of the house "there and then" and pinned him down during a row. The two men never spoke again except through lawyers, but Zaid denied rumours that he had threatened to kill his brother.
He said he knew little about a Swiss bank account containing the proceeds from their father's business in Iraq and would not comment on claims that he attempted to access it using an expired card or tried to fake their father's will.
On Tuesday, the Surrey force said: "Surrey police can confirm that an arrest has been made in France today in connection with the murders of four people near Annecy, southern France in September 2012.
"The arrest has resulted from a line of inquiry in France and is not as a result of the investigation carried out in the UK."