A male biker wanted in connection with the murders of four people in the French Alps could be the killer, the man leading the investigation has said.

Police have released an image of the male motorcyclist as part of their investigation into the murder of a British satellite engineer gunned down with his wife and mother-in-law last year.

Annecy Prosecutor Eric Maillaud said teams have investigated the unusual helmet, of which only "a few thousand" have been made but have so far failed to trace the man.

Mr Maillaud said: "The motorcyclist who we are trying to find was seen by officials from the National Forestry Office a short time before the murders, and in the proximity of the car park where the murders took place. It is completely unknown who he is.

"It could be that he was on a walk and did not see anything; that he is someone who saw something (a witness); or that he is the perpetrator of the murders. All these hypotheses are being considered."

Saad al-Hilli, 50, from Claygate, Surrey, his 47-year-old dentist wife Ikbal and her elderly mother Suhaila al-Allaf, who lived in Sweden, were murdered on a remote forest road near Lake Annecy in southern France on September 5, last year. French cyclist Sylvain Mollier was also killed.

Iraqi-born Mr al-Hilli's eldest daughter Zainab was shot, beaten and left for dead, but survived. Her four-year-old sister Zeena lay hidden under her mother's corpse and was only discovered eight hours after the murders.

The French Gendarmerie have released the artist's impression of a motorcyclist they are trying to trace who was seen riding in the area between 3.15pm and 3.40pm on the day in question, shortly before the four murders took place.

Mr Maillaud added: "His helmet is very particular and there are only a few thousand of these models worldwide."

The lower part of the helmet opens laterally to reveal the lower face, which allowed French police to see that the biker has a beard.

The Annecy prosecutor said that police have been looking for this helmet for the last 15 months, but due to a failure to find it, police have now released a Photofit image of the motorcyclist.

Officers are now appealing for who anyone who might recognise the man in the image to come forward.

He has been described by witnesses as wearing a black or dark-coloured helmet, with a lateral opening at chin level. The only similar helmet that is on the market is made by GPA and is an ISR model, French police said.

Mr al-Hilli's 54-year-old brother, Zaid al-Hilli, from Chessington, south-west London, was arrested in June on suspicion of orchestrating the shootings to settle a family feud and has since been bailed until January.

He has strenuously denied any involvement and accused the French authorities of ignoring details which might reveal that the intended target was the local cyclist also found dead at the scene. The widower has said he would be willing to take a lie detector test to prove his innocence.

Following the killings, speculation was rife about a dispute between the al-Hilli brothers over their inheritance. Zaid has admitted there were tensions over the £825,000 home in Claygate, Surrey, where Saad and his family lived.

The older brother was left his mother's 50% stake in the property after she died from a heart attack in 2003 and claims Saad began demanding that share of the property in 2011.

After his brother allegedly pinned him to a bed, the two did not speak again except through lawyers, but Mr al-Hilli has denied claims that he had threatened to kill his brother.

Surrey Police said their officers are continuing to work closely with their French counterparts on the joint investigation into the murders.

Anyone with information is asked to call Surrey Police on 101 or to call Crimestoppers anonymously on 0800 555 111.