Most Americans are conspiracy theorists when it comes to John F. Kennedy.

A strong majority of Americans continue to believe that the former president's assassination wasn't the work of just one man, though the number believing one man was solely responsible is on the rise.

Over six in 10 Americans say the assassination was part of a broader plot rather than just Lee Harvey Oswald, according to a new Washington Post-ABC News poll. Another 29 percent blame one man alone -- the highest that number has been since the 1960s.

The Warren Commission in 1964 concluded that Oswald was solely responsible.

Another 62 percent also say there was an official cover-up to keep the public from learning the truth about the assassination. Indeed, majorities over the past four decades of polling have consistently sided with the prospect that more than one person was involved in the 35th president's assassination on Nov. 22, 1963.

Americans are far from certain there's a conspiracy at hand. Over half of those who suspect a broader assassination plot say it's based on a hunch, while the rest say they are "pretty sure." Those who say Oswald worked alone are just as tenuous in that belief, with half saying their view is not certain. Altogether, only 44 percent of Americans are sure how Kennedy was killed, whether by “one man” or “a broader plot.”

When a similar question was first asked in September 1966 by the Harris poll, 46 percent suspected multiple actors and 34 percent said Oswald acted alone. One in five were unsure at that time.

The unsettled opinion in 1966 came two years after the Warren Commission report. Subsequent investigations into the assassination have cast doubts on those initial findings while views of a conspiracy have grown.

The Church Committee, a Senate-led investigation in 1976 into the CIA and FBI, concluded that the 888-page Warren Report may have been insufficiently thorough and suppressed key evidence, giving legs to the persistent belief that a cover-up was involved.

By 1979, the U.S. House Select Committee on Assassinations determined that the assassination was "probably" the result of a conspiracy. Indeed, four years after that report, public perceptions of a conspiracy hit their peak at 80 percent.

The potential conspiracy theorists include Secretary of State John Kerry, who recently told NBC News: "To this day, I have serious doubts that Lee Harvey Oswald acted alone. I certainly have doubts that he was motivated by himself."

There may be another shake-up in the beliefs about a Kennedy assassination cover-up when the remaining 1,171 documents from the investigation are unclassified in 2017. Those records, classified by the CIA on national security grounds, may continue to be held if the president deems them to be of continuing national security importance.

The Post-ABC poll was conducted Nov. 14-17 among a random national sample of 1,006 adults, including interviews on landlines and with cellphone-only respondents. The overall margin of sampling error is plus or minus 3.5 percentage points.

Former Dallas Morning News reporter Hugh Aynesworth recounts the "pandemonium" after JFK's assassination. And he tells the Post's Lee Powell about his scramble for a pencil. He was there at Dealey Plaza, at the theater where Oswald was captured, and when Jack Ruby shot Lee Harvey Oswald. (The Washington Post)