The arrest will allow investigators to carry out searches and check the individual's movements around the time of the killing of three members of the Al-Hilli family and a passing cyclist on September 5, 2012, prosecutors in Annecy said.

The individual was detained by police from the Alpine town of Chambery. BFM TV said he was a man who had already been interviewed by police as a witness in the case but never detained.

Saad al-Hilli, a 50-year-old Iraqi-born British tourist in France, was gunned down along with his 47-year-old wife Iqbal and her 74-year-old mother in a woodland car park close to the village of Chevaline in the hills above Lake Annecy.

Each was shot several times in their British-registered BMW estate car and more than two dozen spent bullet casings were found near the vehicle.

The couple's two daughters, aged seven and four at the time, survived the attack, but the older girl was shot and badly beaten.

A 45-year-old French cyclist, Sylvain Mollier, was also killed after apparently stumbling upon the scene.

Further details will only be made public once the man's detention period expires, the prosecutors said.

- Repeated false leads -

Almost a decade after the killings, French and British police have so far failed to make any real progress in the case despite a massive effort involving officers on both sides of the Channel.

During the course of the investigation, several individuals have been detained but without ever being charged.

In 2014, French authorities said that a biker long wanted in connection with the murders had been identified but had no link to the killings.

After a review, authorities also no longer believe former soldier Nordahl Lelandais, who has already confessed to the killing of a hitchhiking soldier and an eight-year-old schoolgirl, could be linked to the killings.

Meanwhile, Saad al-Hilli's brother Zaid al-Hilli, who was arrested in Britain in June 2013 on suspicion of conspiracy to commit murder, was told in early 2014 by British police there was not enough evidence to charge him.

The brothers, born to middle-class parents in Baghdad before the family moved to Britain in 1971, had enjoyed a close relationship. But they fell out over the family house inherited from their mother, who died in 2003.

The Hilli family lived in the Surrey village of Claygate, a leafy suburb outside London.