“The whole right side of his skull was gone. I could look inside his skull cavity. Obviously, it was a mortal wound,” he told a spellbound audience of legal, medical, forensic and investigative experts and the public who packed the university’s Power Ballroom.

Dr. McClelland, now 83 and professor emeritus at the University of Texas Southwestern Medical Center, said that because it was an exit wound, it logically followed that it had been fired from in front of the president’s limousine. And, in turn, that meant a second gunman was involved in the assassination, contradicting the Warren Commission’s finding that there was but one assassin.

The Warren Commission determined that Lee Harvey Oswald acted alone when he fired three times with a high-powered rifle on the president’s motorcade in Dealey Plaza from the sixth floor of the Texas School Book Depository. The commission said that one bullet missed, another went…