The image features a man sporting a goatee and wearing a rare type of motorcycle helmet (see below for full image).

"More than the identikit photo itself, it is the type of helmet that is interesting," said Benoit Vinnemann, the head of investigations for the local gendarmerie.

Saad al-Hilli was gunned down in September 2012 along with his wife and her mother in a woodland car park close to the village of Chevaline in the hills above Lake Annecy.

His two daughters survived the gruesome attack but French cyclist Sylvain Mollier, apparently an innocent bystander, was also killed.

Investigators have struggled to find any leads in the case and are now focusing on the presence of a motorcyclist seen by witnesses near the scene of the shooting.

Witnesses gave a description to police of the motorcyclist from early on in the case but it was not initially released for fear he would go into hiding.

Investigators said the helmet he was described as wearing is a rare model, with only 8,000 made in the colour seen by witnesses.

It is a full-face model that opens from the side to allow conversations without removing the helmet.

Despite checking with distributors, police have so far been unable to track down the helmet's owner.

"This is someone who was close to the scene during a time period that interests us and who may have done or seen something," Vinnemann said. "He is potentially the perpetrator, but mainly a witness."

Police are also searching for a right-hand drive BMW 4×4 spotted close to the scene.

Police have put forward a number of theories for the killing, in particular the possibility of an inheritance dispute involving Hilli's brother Zaid, who was arrested by British police in June.

He was subsequently released and has protested his innocence.

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