Brett Martin said he initially believed there had been a terrible accident as he surveyed the scene in the secluded car park close to Lake Annecy.
The BMW's engine was still revving and its wheels were spinning, he told the BBC. Inside were the bodies of engineer Saad al-Hilli, 50, his dentist wife Iqbal, 47, and her mother.
A fourth body, that of Sylvain Mollier, 45, the French cyclist who apparently stumbled across the attack, lay on the ground.
Close by, Zainab al-Hilli, seven, was "prone on the road, moaning, sort of semi-conscious", Mr Martin said.
The carnage resembled a Hollywood scene, he said.
"It was pretty much what you would imagine a set from (TV crime series) CSI Miami would be like," he added.
"There was a lot of blood and heads with bullet holes in them."
Mr Martin said he set out, as he had on most days that week, for a bike ride from his base at about 2.30pm.
But as he climbed to the top of a hill in the Combe d'Ire forest, near Chevaline, last Wednesday, he was faced with a bloodbath.
"It was the sort of thing you would never in your life expect to come across," he said.
"As I approached the scene, the first thing I saw was a bike on its side. I had seen the cyclist ahead of me much earlier so I thought he was just having a rest.
"As I got a little bit closer, a very young child stumbled out onto the road and at first I thought she was actually just playing with her sibling because she sort of looked, from a distance, as if she was falling over, larking about like a child would.
"However, as I approached her it was obvious that she was quite badly injured and there was a lot of blood on her.
"As I got even closer, I then saw the car with its engine revving and its wheels spinning. It seemed at that moment in time like there had been a terrible car accident."
At the time there was no sign of the al-Hilli's younger daughter Zeena, four, who had been cowering underneath her mother's body during the brutal attack.
Mr Martin said he immediately turned his attention to Zainab, first moving her body from where it lay in front of the car, fearing the vehicle could lurch forward and crush the child.
The little girl appeared "severely injured" and there was "a lot of blood", he said.
He placed her in the recovery position and a few minutes later, she fell unconscious.
Turning to the BMW, he swiftly realised there was nothing he could do for its occupants.
"The thing that struck me was their complete inanimate nature, which was how I assessed really, without breaking into the car and physically handling them, that they were dead," he recalled.
His RAF training meant he was able to keep a clear mind and "take stock" of the situation before deciding on how to proceed, he said.
Mr Martin, who switched off the car's engine, said he then wanted to phone for help but found he had no reception on his mobile.
He realised he would have to leave the scene to make the call but this left him with the "dilemma" of whether to take young Zainab with him, or leave her where she lay.
Fearing that he could do more damage - and possibly kill her - if he lifted her, he set off on his bike.
"The wasn't a very comfortable decision to have to make," he said.
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