Media playback is unsupported on your device Media caption Alps murder witness speaks for first time to BBC Panorama

A witness who claims to have seen the car thought to have been involved in the murders of three members of a British family and a French cyclist in the Alps last September, has spoken publicly for the first time.

A forestry worker has told the BBC's Panorama programme he saw a British dark grey BMW shortly before the attack.

We drive through the sleepy village of Chevaline up a narrow road leading high into the Alps.

Rounding the corner we pull over at a parking spot called Le Martinet, hidden beneath the tree canopy.

The mountains are shrouded in mist and amidst the rush of the stream and chatter of birds, it is hard to imagine the horror of what happened in this idyllic spot.

I was thinking to myself, I wonder if this is going to be painful when I get shot? Brett Martin, Cyclist who discovered the attack

Three members of the al-Hilli family - a father, mother and grandmother - were murdered at point blank range here.

A seven-year-old child was left for dead and her four-year-old sister was so traumatised she was only discovered hiding underneath her dead mother's skirt in the back of the car eight hours after the emergency services arrived.

Only Zaid al-Hilli, the brother of the driver of the car, has ever been arrested on suspicion of conspiracy to murder. He denies arranging the murders and is on bail. He has not been charged.

I have come back to the scene with a British cyclist who discovered the crime to find out exactly what happened.

Brett Martin, a former RAF pilot who has a holiday home in the area, set off for a cycle ride in the early afternoon of 5 September 2012.

"I left the house at about 14:30 [with] no fixed route in my mind," he said.

"It was sunny… [and] quite a peaceful, pleasant afternoon."

He took the same narrow road up the mountain. Ahead of him was a French cyclist. On his way up he was overtaken by a vehicle which he believes was the al-Hilli's car.

According to French police, the Al Hillis reached Le Martinet at about 15:40, having passed the French cyclist. Saad al-Hilli got out of the car with his seven year old daughter, Zainab.

Image caption Brett Martin discovered the killings and has revisited the scene with Panorama

In less than a minute, Saad and his wife Iqbal, from Surrey, her mother Suhaila al-Allaf, and the French cyclist Sylvain Mollier, had been fatally shot.

Zainab had been wounded but the gunman ran out of bullets and clubbed her with his gun. She survived but all she has been able to tell investigators about the killer is that she saw 'one bad man'.

Coming up the hill at about 15:45, Brett Martin saw the motorcyclist escaping.

"He was going at a very slow, abnormally slow speed, and at the time it seemed odd," he said.

When he arrived at the scene, Mr Martin initially thought he had come across a traffic accident. Realising there had been a shooting, he feared for his own life.

"I was thinking is there a hunter, a sniper kind of character, hiding in the trees, maybe shooting from a covered position?

"I was thinking to myself, I wonder if this is going to be painful when I get shot?"

Image caption Panorama has reconstructed the sequence of events leading up to the attack

Mr Martin tried to call the emergency services but could not get a signal. After checking on the little girl he went for help.

No culprit has been found and French police admit they have no evidence as to who the hitman and his accomplices were.

But Panorama has tracked down a witness who suggests the killer was not acting alone.

A forestry worker coming down from a nearby mountain just minutes before the shooting has never spoken publicly before.

He drove me along the same route but does not want to be identified for fear of the killer still at large.

"When I arrived there was a motorbike pulling in to the parking area. I passed the parking and the motorbike was on the left here," he said.

"I remember it well, it was white, white and black, with panniers on either side.

"The rider was all in black [and] his visor was completely closed."

He also saw a British car driven by someone who seems to have been an accomplice.

They had words with him because motor vehicles aren't allowed. So they called out to him and asked him to drive down. They saw his face because he took off his helmet Forestry worker

"He arrived on the left and he passed really quickly. The car was a BMW 4x4, X5, grey metallic, in good condition.

"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."

Ten minutes later, two of his colleagues driving down the same road saw the motorbike up the hill from the parking area beyond where motor vehicles were not allowed to go.

"They told me they passed the motorbike I had passed at Place Martinet - two bends further up.

"They had words with him because motor vehicles aren't allowed. So they called out to him and asked him to drive down. They saw his face because he lifted his helmet...he had a bit of a beard."

This seems to suggest that at least two people acted together - one carrying out surveillance, the other the hitman.

The French prosecutor, Eric Maillaud, has said there is no evidence as to who the hitman or his accomplices were.

In less than a minute, 21 bullets were fired with great accuracy, leading investigators to wonder whether the attack was a contract killing carried out by someone in the criminal underworld.

French police have traced all the vehicles in the area that day except two - the motorbike and the grey four-by-four. They are still searching for them.

The terrible events he witnessed have not stopped Brett Martin from riding his bike up the same road when he visits his holiday home.

But the events of that day will always be with him.

"It's a mystery. Perplexing, unsolved, [it's] one of those Agatha Christie 'whodunit' scenarios."

Panorama: Murder in the Alps, BBC One, Monday 21 October at 21:00 BST and then available in the UK on the BBC iPlayer.