The assassination of John F Kennedy is perhaps one of the most defining moments of the 20th century.

According to official reports, Lee Harvey Oswald was the prime (and only) suspect. The ex-marine had positioned himself next to a window on the sixth floor of the Texas School Book Depository overlooking the Dealey Plaza in Dallas, Texas. From there he shot Kennedy twice with a high-energy Carcano military rifle, once in the neck and again in the back of the head. 

And despite the many, many conspiracy theories cooked up over the years – was there a second shooter? Was it an inside job? Was a Soviet agent involved? – this is most probably exactly what happened. New research, published in Heliyon, has used original footage and digital models to show that Oswald was, indeed, a lone assassin. 

"The classified JFK files were recently released, and the fate of a few remaining files will soon be decided," Nicholas Nalli, Senior Research Scientist at IMSG and author of the study, said in a statement. (Those final few files are due to be released today after the initial deadline was extended in October.)

"Given the current trendiness of news that are not based on facts, the study shows that thorough scientific investigation can make a difference in supporting one theory over another, and I therefore believe that the topic is as relevant today as it was nearly 55 years ago." 

Nalli re-analyzed the Zapruder film, a citizen-shot video clip that captures the very moment Kennedy was hit with the fatal bullets. The film was used as evidence during an investigation by the FBI and again by a Presidential Commission set up by LBJ and led by Supreme Court Chief Justice at the time Earl Warren, but both inquiries apparently missed a crucial piece of information – that is, the forward motion of Kennedy's head when the bullet struck.

This, he says, shows that it came from behind and not, as many have speculated, from a second shooter hiding out in the grassy knolls. 

To prove it, Nalli built a one-dimensional gunshot wound dynamics model that used data on bullet mass, speed, and autopsy measurements to show the effect they had on Kennedy's reaction (specifically, the forward head snap).

The findings show that the movement of Kennedy's head, caught in the Zapruder film, is consistent with the official line: he was shot from behind by an assassin using a high-energy Carcano military rifle from the Texas School Book Depository building.

Nalli does point out this doesn't necessarily prove for a fact the conspiracy theories are wrong – it just makes them a lot less likely.

"The historical fact of the matter is that the US federal government investigations were comprised of upstanding civil servants of high ethical standards who, in spite of difficult circumstances, by-and-large got the basics of the case correct," he explained.