Filmmakers sometimes have to be detectives too, which was the case with the making of documentary “Steve McQueen: The Man & Le Mans,” which world premieres in Cannes Saturday.

The film, which is directed by Gabriel Clarke and John McKenna, tracks the making of McQueen’s motor-racing movie “Le Mans” in 1970. “It is the story of his pursuit of the ultimate racing movie,” Clarke told Variety. The production of “Le Mans” did not go to plan, and the actor’s personal and professional life also started to skid.

Clarke and McKenna found an ally in McQueen’s son, Chad McQueen, who is also a motor-sport fan, and is in Cannes to support the launch.

The star of the documentary is the archive footage the filmmakers tracked down, alongside interviews with Chad McQueen, and Steve McQueen’s producing partner Bob Relyea, who died in 2013. Archive material includes interviews with John Sturges, who was supposed to direct “Le Mans” but quit, and an interview with Steve McQueen when he was dying of cancer.
But the prize material was boxes of rushes from “Le Mans.” These were located thanks to a tip-off from someone who’d worked at CBS’ Cinema Center Studios, which had funded “Le Mans.”

“McQueen shot something like a million feet of rushes, which we called ‘the holy grail’ during pre-production,” Clarke said. “Many of those we spoke to – including Chad – said that all the material had been incinerated. No one knew where they were. But as a result of continued pursuit, the footage was found under a sound stage in one of the old studios where the film was edited in L.A. More than 400 boxes of film, covered in dust. Each one read ‘Le Mans’ along its spine. It was just fantastic news for us.”