Forty years after the event, John F. Kennedy's assassination on Nov. 22, 1963, remains the most popular subject for conspiracy theories in America, says a UC Davis scholar who is writing a book on such theories during the 20th century.

In fact, only 10 percent of Americans believe Lee Harvey Oswald was acting alone when he shot the 35th president, says Kathryn Olmsted, author of "Challenging the Secret Government: The Post Watergate Investigations of the CIA and FBI."

"Throughout their history, Americans have been concerned about excessive government power, and you can see conspiracy theories dating back to the founding of the nation," Olmsted says. "But it isn't until World War I, when you get a really powerful government, that conspiracy theories became more extreme and widespread."

Conspiracy theories about JFK's murder didn't really gain steam until the Church Committee in 1975 revealed government cover-ups in the original 1964 Warren Commission report on the Kennedy assassination. In particular, the public learned about assassination attempts on Cuba's dictator, Fidel Castro.

"When people learned that the Warren Commission deliberately covered up a lot of facts, that provided fodder for a number of conspiracy theories. People thought, 'If government is covering up these assassination plots about Castro, what else was it capable of covering up?'" Olmsted says.

Tracking JFK conspiracy theories is now a major hobby, with hundreds of books offering various theories of who killed the president, ranging from the FBI and CIA to the Mafia and Lyndon B. Johnson.