French police investigating the murder of a British family at an Alpine beauty spot have issued a sketch of a man seen near the scene.

The motorcyclist, described as bearded and wearing a full-face helmet, was spotted by two witnesses above the parking area at the top of a narrow country road near the village of Chevaline where Saad al-Hilli, his wife, Iqbal, and mother-in-law, Suhaila, were gunned down on 5 September last year.

The victims were shot in the head. A French cyclist, Sylvain Mollier, who detectives believe stumbled upon the scene, was also killed.

French investigators, who suspect the family may have been targeted by a professional hitman, have known of the bearded motorcyclist since the beginning of the inquiry. They say he was wearing an unusual helmet, produced by the company GPA, of which only 8,000 were made.

However, searches at the homes of known owners of similar helmets and monitoring of internet sales sites such as eBay have yielded no information.

Prosecutor Eric Maillaud, who is overseeing the inquiry, told French journalists on Monday: "It's always useful to see if anyone might recognise the motorcyclist because he could have seen something that might interest us."

A second motorcyclist was reportedly seen in the area after the killings, but did not appear to be fleeing the scene, he added.

Saad al-Hilli, 50, his 47-year-old wife and her mother, 74, were on holiday in the area, near Annecy, when they were killed.

The couple's elder daughter, Zainab, then aged seven, was pistol-whipped by the attacker and left for dead with serious head injuries. Her sister, Zeena, four, cowered under the skirt of her dead mother in the rear of the family's BMW estate car for eight hours before she was found by police.

Detectives are still hunting a BMW X5, a dark-coloured, four-wheel drive vehicle, spotted in the area. The car stood out as it was a right-hand drive, but investigators say they are not yet linking the car and the motorcycle.

A French hotline has been opened on 0800 002 950.

The photofit shows a man with a moustache and beard wearing a black or dark coloured helmet that is unusual because it has a lateral opening at chin level. Police said they were appealing for "anyone who recognises the man" to call them.

Detectives had held back on issuing a portrait of the man they want to speak to, hoping to be able to trace him through his relatively rare helmet.

The man was seen by members of the National Forests Office (ONF) near where the bodies of the murdered al-Hilli family members were discovered by a British cyclist only moments after they had been killed. "What is more interesting then the photofit itself is the type of helmet," Lt Col Benoît Vinnemann, the gendarme heading the investigation, said.

Police said the helmet was produced in Italy by French specialist company GPA - based in the south of France - and was a "ISR type", unusual because it has a hinged opening at chin level to allow the wearer to hold a conversation without removing their helmet. Only 8,000 models in black or a dark colour were made by the company.

"This man was riding along the Combe d'Ire at Doussard - Chevaline between 3.15pm and 3.40pm on 5 September 2012. The only helmet on the market that matches the description we have is the ISR-type GPA helmet," police said in a statement on Monday. Despite checking with distributors, police have been unable to trace the owner.

More than a year after the assassinations, police are still unable to establish whether the al-Hillis were killed as a result of a family dispute over an inheritance, over the family's Iraq connections, as a result of industrial espionage or by a lone gunman unconnected to the victims.