The Nix film is the Betamax of JFK assassination movies. Historically, it has been overshadowed by Abraham Zapruder’s film, which captured the 1963 shooting in horrific detail, and yet Orville Nix’s home movie could actually reveal more – at least for conspiracy theorists. This week, the Nix film itself became part of those conspiracy theories, when Nix’s granddaughter, Gayle Nix Jackson, took the step of suing the US government for $10m for the return of the original film. Where is it? Er, nobody knows.

What makes Nix’s film so special is that it was shot from the opposite side of the president’s motorcade to Zapruder’s (if you enhance the images, you can actually see Zapruder in it). Thus, it captures the famous “grassy knoll”, the area where sceptics believe JFK’s real killer was concealed, as opposed to Lee Harvey Oswald up in the Texas School Book Depository.

The chronology is confusing. A week after the assassination, Nix passed a copy of his film to the FBI and sold the original to the UPI press agency for $5,000. Alongside Zapruder’s and other films, his footage became a key piece of evidence in official investigations, including the Warren Commission in 1964 and the House Select Committee on Assassinations in 1978. The latter borrowed the original Nix film from UPI, and claimed to have returned it, but nobody ever saw it again.

In the intervening half-century, innumerable experts have pored over copies of Nix’s grainy 8mm footage, and like David Hemmings in the movie Blow-Up, theyhave “seen” things in it: a gunman’s silhouette on the grassy knoll; puffs of gunsmoke; discrepancies with Zapruder’s film. In 1966, Nix himself fanned the flames, telling an interviewer that when the FBI gave him back a duplicate of his movie, some frames were “missing” or “ruined”. He also claimed he thought the shots had come from the grassy knoll area that day, and that “most everyone” thought the same.

Few people still subscribe to that theory, but Nix’s granddaughter is one of them. Last year, she wrote a book on the Nix film’s strange history, which she hoped would “underscore the fact that there was and is a conspiracy to withhold the truth”. If there isn’t one, the US government has done a pretty shoddy job of putting the rumours to bed. And it continues to do so. In 2013, the Obama administration declassified hundreds of millions of CIA documents, but some pertaining to the Kennedy assassination are reportedly being kept back. We won’t see them until at least 2017. Assuming they don’t get lost in the mean time.