Many of us have lived through some red letter dates in American history, events that are so seared in our memories that we know exactly where we were and what we were doing when they occurred.

Most seniors from the Greatest Generation, who endured the Great Depression and then World War II, can recall where they were when Pearl Harbor was attack by the Japanese on Dec. 7, 1941. Baby Boomers can remember what they were doing on Nov. 22, 1963 when President John F. Kennedy was assassinated. Another unforgettable day was Sept. 11, 2001, when terrorists used hijacked airliners to attack American soil.Inevitably, these world-shaking events invite conspiracy theories. These are explanations proposed that accuse two or more people, a secret group or alliance rather than an individual or isolated act. The Kennedy assassination is often seen as the beginning of the modern "conspiracy theory" movement. The Warren Commission's conclusion that Lee Harvey Oswald acted alone simply opened the floodgates for more questions.A Gallup poll released last week found that 61 percent of us still believe the assassination was part of a larger conspiracy, rather than the work of a lone gunman while only 30 percent believe Oswald acted alone.Having spawned more than 2,000 books, the Kennedy shooting stands as the single most studied non-religious event in history. Author James Pierson explains that our obsession with the charismatic president "demonstrates that the assassination was never fully digested by the generation that lived through it."Those people who were eyewitnesses to the assassination on Nov. 22, 1963, are disappearing. Nearly 20 years ago, I was fortunate to interview two persons with ties to the historic event - Dallas Police officer Nick MacDonald, who fought with and arrested Oswald in the Dallas theater a short time after the assassination, and Earl Ruby, youngest brother of Jack Ruby, who gunned down Oswald two days after the assassination in the basement of the Dallas police station. MacDonald died in 2005 and Earl Ruby died in 2006.Both men felt Oswald acted alone and was the only gunman that day. I still count myself as part of the more than half of the adult population in the United States who remain unconvinced that Oswald acted alone. In trying to explain the events in Dallas 50 years ago, I invoke the words of Winston Churchill who, in trying to explain Russia's actions before World War II, called them "a riddle wrapped in an enigma shrouded in mystery."By Jim Zbickeditor@tnonline.com