The detention of the unnamed man has raised hopes of a breakthrough in one of the country's most notorious murder cases that saw three members of a British family gunned down in front of their two young daughters.

A French cyclist was also killed nearby on a remote mountain road near Lake Annecy.

Local newspaper Le Dauphine Libere identified the arrested man as a mystery motorcyclist seen near the scene of the crime who was cleared in 2015 after a more than two-year hunt by police to find him.

Local prosecutor Line Bonnet declined to confirm the report when contacted by AFP on Thursday, but said that the individual was among four witnesses "returned to the scene" to participate in a reconstruction just over three months ago.

She wrote on Twitter that his detention had been extended, giving police another 24 hours to question him and potentially bring charges.

French investigators have continued to work on the more than nine-year-old cold case, but have always struggled to identify a motive for the killings of the Al-Hilli family.

Saad al-Hilli, a 50-year-old Iraqi-born British tourist, 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.

- 'Cleared in 2015' -

A lawyer for the arrested man said on Wednesday evening that the detention was unjustified as his client had already been "cleared in 2015" after being spoken to as a "mere witness".

Lawyer Jean-Christophe Basson-Larbi told reporters that his client's position remained he was in the area at the time and never crossed paths with the Al-Hilli family.

After being questioned in 2015, the motorcyclist was said by the local prosecutor to be "the head of a business in the Rhone-Alps region, with a good reputation and above any suspicion, who had gone to do some paragliding and was on his way home."

Police eventually tracked him down after releasing an image showing him with a goatee and wearing an unusual type of helmet.

He told police that he had not come forward as a possible witness because he had not been paying attention to the media.

He was tracked down after police gathered all 4,000 mobile phone numbers logged in the area on the day of the murders and began ringing each one.