In 1961, the U.S. attacked newly socialist Cuba in the unsuccessful Bay of Pigs Invasion.

In 1962, Soviet Union leader Nikita Khrushchev allied his country with Cuban Prime Minister Fidel Castro and constructed missiles on Cuban soil pointing at America — threatening to fire until the U.S. promised never to invade again.

In 1963, the president of the United States was assassinated.

Coincidence?

“When Castro chose an alliance with the Soviet Union, it was more than our intelligence service could stand … it’s 90 miles off our shores,” said Bryan Ghent, an expert in the assassination of John F. Kennedy at Winthrop University.

So after JFK was shot, some couldn’t help but make a connection.

“Right wing activists very shortly after the assassination started blaming communism and Castro,” said John McAdams, the author of “JFK Assassination Logic: How to Think about Claims of Conspiracy.”

Both Castro and Krushchev provide a path into what Ghent calls the “wonderful rabbit hole” of speculating and circular thinking around who killed Kennedy.

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According to a recent Gallup poll, 61 percent of Americans believe that Lee Harvey Oswald, the man ID'd by the Warren Commission as the lone gunman who killed Kennedy, did not act alone or at least conspired with a person or group of people to kill the president on Nov. 22, 1963. While a majority believes that the mob or sects of the U.S. government were involved, 5 percent believe Castro was to blame, and 3 percent continue to implicate the Soviet Union.

As the nation observes the 50th anniversary of John F. Kennedy’s death, NBC News explores some of the most compelling conspiracy theories that have fueled books, investigations and countless conversations about Kennedy’s death for the last half-century.

The Soviet connection

“Russia and Cuba have fascinating links to the assassination, but it begins with Lee Harvey Oswald,” Ghent said.

Immediately following a three-year tour in the Marines, Oswald lived in the Soviet Union from 1959 to 1962, intending to defect. He met his Russian wife, Marina Prusakova, whose uncle was an official in the Russian Interior Ministry, and “was surrounded by a lot of KGB informants during his time there,” Ghent said.

“The KGB denies that,” but “of course they would,” he added.

Some speculate that Oswald traded military secrets for a position in the KGB, and then returned to the U.S. upon its orders to kill Kennedy, Ghent said. Most who say Oswald traded U.S. intelligence for the good will of the KGB believe that he was recruited to be a spy when he was stationed near a CIA base in Atsugi, Japan in 1957, Ghent said.

The U.S. government has always denied that Oswald was an agent.

Still, others think that Oswald “was replaced by a Russian agent who looked like him in order to assassinate our president,” said Ghent. Oswald’s body was exhumed in 1981 to prove that he was the same man that entered the Marines.

“The teeth matched,” Ghent said, “but the body was missing a surgical scar that was on his military records.”

Some people believe Oswald is alive and well and living in Ohio, McAdams said.

The Cuba connection

Still, some experts say Oswald had a more direct relationship to Castro and was a strong believer in Castro’s vision for Cuba.

“Castro was the only person in the world who Oswald held in high esteem publicly,” said Ed Epstein, author of “The JFK Assassination Diary.”

Oswald was arrested for handing out pro-Castro leaflets in New Orleans during his time involved with a group called Fair Play for Cuba. Shortly after that he spoke out in a radio interview and later, a debate, unabashedly defending the Cuban leader and his Marxist beliefs.

“Castro was charismatic and Oswald was looking for new adventure,” explained former CIA agent Brian Latell, author of a new book, “Castro's Secrets: The CIA and Cuba's Intelligence Machine.”

Latell doesn’t think that Castro directly ordered Kennedy’s death, but believes he had knowledge of Oswald’s plan and purposefully didn’t stop it.

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“The Cuban intelligence had quite a big file on Oswald,” even though Castro denied having ever heard of him, Latell said.

Just seven weeks before the assassination, Oswald took a trip to Mexico City to visit the Soviet Embassy there. He also went to the Cuban Embassy in an attempt to gain access to Cuba “to fight in the mountains with Fidel,” according to Latell. Oswald allegedly left the Cuban Consulate yelling, “I’m going to kill Kennedy.”

“That report is solid gold,” Latell said.

What does Latell think? Officials at the consulate took advantage of Oswald and inspired him to assassinate Kennedy, he said.

Epstein isn’t convinced that Cuban intelligence persuaded Oswald, but through research has found that Kennedy's plot to take out Castro was moving "completely in tandem" with Oswald’s plot to kill Kennedy.

“If they had called off the plots against Castro, Oswald would not have shot Kennedy,” he said.

Others don’t believe Oswald was at the Cuban Embassy at all, but think a lookalike was sent to the embassy to implicate Oswald and Castro, Ghent said.

“Can you see why this is so endlessly fascinating?” he asked. “Every door opens ten more until you forget how you got there.”

McAdams had a different take on the allure of conspiracy. “Whoever you dislike, you blame them for killing Kennedy,” he said.

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'So consequential an act': 50 years later, JFK conspiracy theories endure