Bonar Menninger was a journalist on the Kansas City Business Journal. He took a keen interest in the assassination of John F. Kennedy and for many years worked on a book on the case with Howard Donahue, a retired ballistics expert from Towson, Maryland,
Menninger's book, Mortal Error: The Shot that Killed JFK, was published in 1992. In the book Menninger claims that Kennedy was killed by Secret Service agent George Hickey, who, riding in the car directly behind the president, tried to stand up on the backseat of the car, lost his balance, and accidentally discharging his gun into the back of Kennedy's head.
The book was based on the following evidence: (1) S. M. Holland saw Hickey lose his balance when he stood up during the firing; (2) AR-15 rounds are encased in thin copper and tend to break up upon impact, as did the shot that struck John F. Kennedy in the head; (3) A Mannliher-Carcano bullet would not break up when it hit a target; (4) Ralph Yarborough and other witnesses smelled gunpowder soon after the shooting, indicating that at least one shot had been fired from street level; (5) Two witnesses, Austin Miller and Royce Skelton, thought one of the shots came from near the presidential limousine and (6) Howard Donahue argued that the bullet's trajectory that hit Kennedy in the head suggested it came from Hickey's gun. sidearms."
In April, 1995, George Hickey sued St. Martin's Press about what was said about him in Menninger's book, Mortal Error: The Shot that Killed JFK. His lawyer, Mark S. Zaid: "We're trying to stop this now while Hickey's still alive... He doesn't want his grandchildren growing up and hearing other children say, Hey, your grandfather killed the president of the United States." According to Zaid: "We settled the case then but only if it included an apology from the publisher that would send the message to most reasonable people that the theory was flawed."
George Hickey died in 2011. Aware that he could no longer be sued, Bonar Menninger worked with Colin McLaren, a veteran Australian police detective, to make a documentary, JFK: The Smoking Gun, repeating the claims.