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 French cyclist on September 5th, 2012, prosecutors in Annecy said.

The individual, whose age and gender were not specified, was detained by police from the Alpine town of Chambery.

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 used bullet casings were found near the vehicle.

The couple’s two daughters, aged seven and four at the time, survived the gruesome 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.

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.