Who killed JFK?Fifty years after the death of the 35th president, thats still a provocative question for many.Conspiracy theories began swirling almost immediately after President John F. Kennedy was assassinated in Dallas on Nov. 22, 1963, and have never really stopped.A spate of new books re-examining that moment in anticipation of the 50th anniversary has revived some theories, tried to squelch others and found intriguing new details of botched investigations or deliberate concealment by authorities.Theres a ready audience: Sixty-one percent of Americans believe that Lee Harvey Oswald did not act alone in killing the president, according to a Gallup poll released Friday. While the percentage of those who believe in a conspiracy is the lowest since the late 1960s, it confirms the publics ongoing doubts about the lone gunman theory.The likely conspirators?The poll found that 13 percent believe the Mafia and 13 percent think the federal government was involved; 7 percent named the CIA; 5 percent each for Cuban leader Fidel Castro, special interests and political groups; and 3 percent each for the Ku Klux Klan, Vice President Lyndon Johnson and the Soviet Union.The random-sample poll of 1,039 people 18 and older was conducted Nov. 7-10. It has a margin of error of 4 percentage points.Belief in a conspiracy has endured for nearly 50 years of polling. Doubts also persist about the investigative findings of the Warren Commission, which was created by Johnson after he became president and was led by Chief Justice Earl Warren.It is a deep-seated belief that no single man could have committed what some consider the crime of the century that has been part of the American psyche since the 1960s and that got a Hollywood boost from director Oliver Stones conspiracy-fueled 1991 film JFK.Kerry weighs inBut its also a belief that no one speaks about too loudly, as Secretary of State John Kerry discovered this month when he said publicly that he doesnt think Oswald had acted alone, only to clam up within days.To this day, I have serious doubts that Lee Harvey Oswald acted alone, Kerry told NBC News Tom Brokaw for a 50th anniversary package. I certainly have doubts that he was motivated by himself.Kerry touched on several theories that have swirled around the assassination: Was more than one gunman involved? Besides the ones from Oswalds perch on the sixth floor of the Texas School Book Depository, did more shots come from the grassy knoll at Dealey Plaza? Did Cuba and the Soviet Union communist nations furious about being pressured to remove Soviet missiles from Cuba figure in Oswalds action?Im not sure if anybody else was involved, Kerry said. I dont go down that road with respect to the grassy knoll theory and all of that. But I have serious questions about whether they got to the bottom of Lee Harvey Oswalds time and influence from Cuba and Russia.Oswald, a former Marine, defected to the Soviet Union for years and married a Russian woman before returning to Texas. He was also considered a Cuban government sympathizer who, seven weeks before the shooting, was in Mexico City trying to get a visa to Cuba.Kerry told Brokaw that he doesnt agree with the popular theory that the CIA was behind the assassination. Some skeptics of the Warren Commission report maintain that the CIA was humiliated by Kennedys refusal to provide air cover for the Bay of Pigs invasion, a failed agency-backed effort to invade Cuba and overthrow Castro.But when Kerry then appeared on NBCs Meet the Press, presumably to talk about foreign policy, the former Massachusetts senator refused to respond to questions about the assassination.Time in Mexico CityBut others have weighed in.In A Cruel and Shocking Act: The Secret History of the Kennedy Assassination, former New York Times reporter Philip Shenon opens with a revelation that the Navy pathologist who examined Kennedys body burned the original autopsy report because it contained drops of the presidents blood.The book also has new details about Oswalds time in Mexico City, including meetings with the Soviet KGB, which the CIA allegedly hid from the Warren Commission.In Mexico City there were a lot of people who wanted to see Kennedy dead who met with Oswald, Shenon said.Still, Shenon isnt pushing a conspiracy theory.All the most credible evidence points to Oswald as the shooter of the president and the killer of Tippit, he said of Dallas police officer J.D. Tippit, who was killed while trying to detain Oswald. My suspicions are who else knew and if he was encouraged to do it.After extensive research, University of Virginia professor Larry Sabato concludes in The Kennedy Half-Century that the evidence of a fourth gunshot and thus a second gunman is wrong. Studies found that recordings from an open microphone on a Dallas police officers motorcycle dont include the sound of any shots, Sabato said, because it was too far away.The debate over Nov. 22 will likely never end, he said upon the books release at Washingtons Newseum last month, adding that the Warren Commission has led to 50 years of unending suspicions and cynicism.While hes confident that no evidence of a fourth shot shows up on recordings, Sabato doesnt close the door to all conspiracies.Amoral psychopathRepublican political consultant Roger Stones The Man Who Killed Kennedy: The Case Against LBJ is a conspiracy-laden work that brings the CIA, the FBI, the Mafia and Texas oilmen together under a manipulative Johnson, who wants to be president. Johnson died in 1973.This guy is an amoral psychopath, Stone said in an interview.A Texan, Johnson insisted on the trip to the Lone Star State despite Kennedys reluctance to go to a hard-core conservative area, Stone said. Johnson also allegedly thought hed be dumped from the ticket when Kennedy faced re-election in 1964.I believe Johnson was the yoke of the conspiracy, Stone said. He also includes disgruntled mob bosses whod given money to Kennedys father to help win the 1960 election, only to be investigated by an aggressive Attorney General Robert Kennedy, the presidents brother. Stone also maintains that there were multiple shooters.Julian Read, the press aide to Gov. John Connally who was wounded in the car that was carrying the president and first lady Jacqueline Kennedy disputes the notion of a conspiracy in his book, JFKs Final Hours in Texas: An Eyewitness Remembers the Tragedy and Its Aftermath.No one could keep a secret that long if there was a conspiracy, he quotes Connally as saying.The assassination had a searing impact on Americans, Read said, and now people want to find peace with it. Its the most universally shared experience in our history, Read said.Two nobodiesHugh Aynesworth was there, too.Ive never been a conspiracy theorist, said the veteran Texas newspaperman, who has tracked down numerous conspiracies throughout his career. His book November 22, 1963: Witness to History has been updated for the anniversary.Aynesworth was at Dealey Plaza that day.I heard three definite, distinct shots, he said, as did those trained observers, police and reporters.Then he went to Dallas Oak Cliff neighborhood after hearing a report on the police scanner and arrived just after Tippit was shot.After that, Aynesworth dashed to a theater and watched police arrest Oswald. Dallas nightclub owner Jack Ruby shot Oswald to death two days later.He called the theory that Johnson played a role in Kennedys death totally off the wall. But hes not surprised that so many different suspected plots have endured for so long.We all love a mystery, he said. We dont feel comfortable with two nobodies changing the course of history like Ruby and Oswald did.