Zapruder film: A 26-second home video by Abraham Zapruder is the clearest recording available of the moment Kennedy was shot. It has been thoroughly pored over in hopes of finding the slightest of clues. The film is now too brittle to run through a projector, but in 1999, the government paid Zapruder's heirs $16 million for it.

The "magic bullet": On Sept. 27, 1964, government investigators released an 888-page report on the assassination, forming the official explanation of the events. It concluded that Oswald acted alone, and that Jack Ruby acted alone in killing Oswald two days later. It placed no blame on suggested conspirators like communists, foreign governments, the U.S. intelligence community or right-wing conservatives. Among its more controversial contentions was that a single bullet — derisively referred to as a "magic bullet" — struck both Kennedy and Connally, who survived the shooting.

Lee Harvey Oswald: Oswald, a former Marine, fired three shots from the Texas School Book Depository, the Warren Commission concluded. He denied that he had shot the president, calling himself a "patsy." He was arrested hours later after shooting a police officer and ducking into the nearby Texas Theatre.

Jack Ruby: Two days after the presidential assassination, Oswald was being transferred from a city jail to a county jail when Ruby stepped out from a crowd and shot him at close range as millions of people watched on live television. He died in jail in 1967.

Grassy knoll: A recent poll by FiveThirtyEight found that 33 percent of Americans believed one person was solely responsible for the assassination, while 61 percent believed others were involved. A poll in 1988 found that 13 percent of Americans believed Oswald acted alone. Skeptics say it would have been impossible for Oswald to fire fast enough to hit both Kennedy and Connally, or for the same "magic bullet" to strike both men. (Connally said he thought he was struck by a separate bullet.) The explanation: a second shooter on what has become known as the "grassy knoll" ahead of the motorcade.

Umbrella Man: It was a beautiful day in Dallas. So why was one man holding up an umbrella? The man, who can be seen in the Zapruder film, became the object of fascination. Could he have been signaling messages to gunmen? Was his umbrella rigged with some kind of weapon? Josiah Thompson, a researcher, gave the mysterious figure a name: Umbrella Man. "The only person under any umbrella in all of Dallas, standing right at the location where all the shots come into the limousine," he said in 2011. "Can anyone come up with a non-sinister explanation for this?" Umbrella Man could. Louie Steven Witt testified in Washington in 1978 that his umbrella was meant to protest the Nazi-appeasement policies of Joseph Kennedy, the president's father. The elder Kennedy supported British Prime Minister Neville Chamberlain, who often carried an umbrella, and he hoped that the president would get the message.

Mexico City: Oswald visited Mexico City for six days shortly before the assassination. He said he was there to obtain visas from the Cuban and Soviet Union embassies. But some people suspect that he worked with other people there to plan the attack, and his actions during his time there remain mysterious.

New York Times