The assassin behind the quadruple murder that saw a British man and two members of his family shot dead in the French Alps may have been assisted by an accomplice driving a British vehicle, a witness account suggests.

Saad al-Hilli, 50, was killed alongside his wife and mother-in-law near the village of Chevaline in the Haute Savoie last year. The body of local cyclist, Sylvain Mollier, 45, lay nearby.

But the motive for the murder, which saw Saad’s seven-year-old daughter pistol-whipped and her four-year-old-sister hide under the skirt of her dead mother for nearly eight hours, remains a mystery, despite an exhaustive police investigation with inquiries stretching as far as Iraq and US.

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A new investigation to be broadcast tonight will suggest that two people acted together – one as surveillance, and the other as an assassin. The theory relies on the account of a Forestry Commission worker who was in the area minutes before the murder.

The man, whose identity is protected, claims to have seen a man on a black and white motorcycle and, shortly afterwards, a right-hand drive car driven by a man he believes was an accomplice.

He told the BBC’s Panorama: “The car was a BMW 4x4, X5, grey metallic, in good condition, clean, it looked pretty new. It was a right-hand drive, English. I didn’t get much of a look at him but the driver was slightly bald and he had dark skin, no glasses.” The claims were last night corroborated by Surrey Police. Despite a public appeal earlier this year, the car has not yet been traced.

Meanwhile in an emotional testimony, Mr Hilli’s brother, Zaid, has accused French police of a “cover-up”. The 54-year-old who remains the only arrest in connection with the case, said he believes that the cyclist, Mr Mollier was the intended target of the attack.

“Sylvain Mollier was involved in family disputes and was an outsider to [his] rich family,” he told the BBC. “There is something more to it locally … most crime has local roots.”

The two brothers are known to have been in a dispute over their father’s legacy. But while Zaid admits there was friction between the two he denies any involvement in his murder.