"JFK: The Smoking Gun" will air on the Reelz Channel on Nov. 3, just weeks before the 50th anniversary of the assassination on Nov. 22.
The documentary is based on the 1992 book "Mortal Error: The Shot that Killed JFK" by Bonar Menninger, who worked with Australian detective Colin McLaren over the course of four years.
McLaren and Menninger allege an inexperienced Secret Service agent named George Hickey, who was in the follow-up car behind the president's vehicle, tried to respond after Oswald fired the first shot.
When the car lurched forward, Hickey's inexperience caused him to fire and accidentally hit Kennedy with the bullet that actually killed him, they said.
"There was no conspiracy," McLaren said. "We're just saying a second shooter fired the fatal round."
Documentary producer Michael Prupas said: "The only conspiracy was the subsequent coverup."
McClaren said he came to his conclusions based on autopsy photos, looking at the trajectory of the fatal shot and the size of the "blow-out effect" on Kennedy's head, which McClaren said was inconsistent with Oswald's location and ammunition.
Hickey sued St. Martin's, the book publisher that produced "Mortal Error," in 1995, but a judge said the suit came too long after the book's publication. After Hickey died in 2011, Menninger began working with McClaren to produce the documentary.