WASHINGTON — Who killed JFK?

Fifty years after the slaying of the nation’s 35th president, that’s still a provocative question for many.

Conspiracy theories began swirling almost immediately after President John F. Kennedy’s assassination 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.

There’s a ready audience: Sixty-one percent of the American people believe that Lee Harvey Oswald did not act alone in killing the president, according to the most recent Gallup poll, released Friday. While the percentage of those who believe in a conspiracy is the lowest since the late 1960s, it confirms the public’s 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 believe Cuban leader Fidel Castro, “special interests” and political groups were responsible; the Ku Klux Klan, then-Vice President Lyndon Johnson and the Soviet Union each drew 3 percent.

The random-sample poll of 1,039 people 18 and older was conducted Nov. 7-10. It has a margin of error of plus or minus 4 percentage points.

The belief in a conspiracy hasn’t diminished in nearly 50 years of polling. Doubts also persist about the findings of the Warren Commission, which was created by Johnson, after he became president, to investigate the assassination and was led by Supreme Court Chief Justice Earl Warren.

It is a deep-seated belief — that no single man could commit what some consider the crime of the century — that’s been part of the American psyche since the 1960s and that got a Hollywood boost from director Oliver Stone’s conspiracy-fueled 1991 re-creation, “JFK.”

But it’s also one that no one speaks about too loudly, as Secretary of State John Kerry discovered earlier this month when he said publicly that he didn’t 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 of the theories that have swirled around the assassination: Was more than one gunman involved? Beside Oswald’s 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 former Union of Soviet Socialist Republics — communist nations furious at being pressured to remove Soviet missiles from Cuba — figure in Oswald’s action?

Oswald, a former Marine, defected to the USSR for several years and married a Russian woman before returning to Texas. He was also considered a Cuban government sympathizer who, seven weeks before the Dallas shooting, was in Mexico City trying to get a visa to Cuba.

Kerry told Brokaw that he didn’t agree with another popular theory that the CIA was behind the assassination. Some skeptics of the Warren Commission report maintain that the Central Intelligence Agency was humiliated by Kennedy’s refusal to provide air cover for the Bay of Pigs plan, a failed effort that the agency backed to invade Cuba and overthrow Castro. But when Kerry then appeared on NBC’s “Meet the Press,” presumably to talk about foreign policy issues, the former Massachusetts senator refused to respond to questions about the assassination.

But 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 his book with a revelation that the Navy pathologist who examined Kennedy’s body burned the original autopsy report because it contained drops of the president’s blood. The book also has new details about Oswald’s time in Mexico City, including meetings with the Russian KGB, which the CIA allegedly hid from the Warren Commission investigation.

“In Mexico City there were a lot of people who wanted to see Kennedy dead who met with Oswald,” Shenon said.

Still, Shenon isn’t 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 trying to detain Oswald. “My suspicions are who else knew and if he was encouraged to do it.”

University of Virginia professor Larry Sabato concludes in “The Kennedy Half-Century” after extensive research that the “evidence” of a fourth shot — and therefore a second gunman — instead of the three shots witnesses heard was wrong. Studies of recordings from an open microphone on a Dallas police officer’s motorcycle don’t include the sound of any shots, said Sabato, because it was too far away.

“The debate over Nov. 22 will likely never end,” he said at the book’s release at Washington’s Newseum last month, adding that the Warren Commission has led to “50 years of unending suspicions and cynicism.”

While he’s confident that no evidence of a fourth shot shows up on recordings of the incident made at the time, Sabato doesn’t close the door to all conspiracies.

Republican political consultant Roger Stone’s “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 then-Vice President 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 Kennedy’s reluctance to go to a hard-core conservative area, Stone said. Johnson also allegedly thought he’d be dumped from the ticket when Kennedy faced re-election in 1964.

“I believe Johnson was the yoke of the conspiracy,” Stone said, in which he included disgruntled mob bosses who had given money to Kennedy’s father to help with the 1960 election, only to be subsequently investigated by an aggressive Attorney General Robert Kennedy, the president’s brother. Stone also maintains there were multiple shooters.

Julian Read, the press aide to Texas 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, “JFK’s 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 having said.

To the author, the assassination had a searing impact on Americans, and now people want to find peace with it.

“It’s the most universally shared experience in our history,” Read said.