Former British air force pilot Brett Martin had been out for a cycle ride last Wednesday when he came across another cyclist and three members of a holidaying British family who had been shot dead on a remote mountain road in the Annecy lake area.
"It seemed to me just like a Hollywood scene. And if someone had said 'cut' and everybody got up and walked away that would have been it. But unfortunately it was real life," said Martin, in an interview with BBC TV.
The victims in the BMW car were Saad al-Hilli, an Iraqi-born engineer, his wife and mother-in-law.
The brutality and unexplained nature of the killings, which police say were carried out using a single semi-automatic weapon, have intrigued media in Britain who have given the story blanket and prominent coverage.
French prosecutors have named Hilli's work in the aerospace industry or a family feud as possible motives.
"There was a lot of blood and heads with bullet holes in them. It's the sort of thing you would never in your life expect to come across," said Martin.
The first thing he said he saw as he approached the scene was a bike lying on its side and a young child, Hilli's seven-year-old daughter Zainab, who had stumbled out into the road.
He initially thought she was playing but, as he got closer, realised she had serious head injuries and was covered in blood.
"She was prone on the road, moaning, semi-conscious and she was lying in a position that was in front of this car with its wheels spinning," said Martin.
"She was very severely injured because she was in and out of consciousness."
He moved her out of the path the car before turning to the cyclist, before quickly deducing he was dead.
At first, he thought there had been a terrible car accident as the vehicle's engine was still running with the wheels spinning vigorously. But as he went to turn off the ignition, he noticed bullet holes in the windows.
"It became fairly evident that the injuries of the people inside didn't match what one would think people would be like from a car accident in a car park," he said.
He did not see Hilli's other daughter, four-year-old Zeena, who was hiding inside under the legs and skirt of her dead mother. She was only found unscathed, almost eight hours after police arrived.
Martin said he had become anxious, fearing there was a crazed gunman on the loose but could not phone for help as his mobile phone had no signal.
Left with the dilemma of what to do with the wounded Zainab, he made the difficult decision to leave her behind and seek help, rather than risk causing her further injuries or even death by trying to take her with him.
Zainab, who suffered serious skull fractures after being shot and beaten, came out of a medically-induced coma on Sunday and will be questioned by police as soon as she is fit.
French investigators, who said about 25 gun shells had been retrieved from the area, travelled to Britain on Thursday to liaise with British detectives who have been searching the Hilli family home in a leafy village in Surrey, south of London.
French prosecutor Eric Maillaud told reporters at a Surrey police station they believed "in all likelihood the origins, causes and explanations are here in this country".
