The Zapruder film, a total of 486 frames lasting 26.6 seconds, was not the only visual record of the murder. Home movies providing insights were also taken by other witnesses, Orville Nix and Marie Muchmore. But Mr. Zapruder, who died in 1970, happened to be standing at a spot on the motorcade route where he was able to film the horror in full. The passage of more than half a century has not diminished the shock power of his footage.

From almost the moment the shots were fired, conspiracy theories flourished: Oswald did not act alone, or was not involved at all. A plot to kill Kennedy was hatched by the Mafia. Or it was by the Central Intelligence Agency. Or by Cubans. Or by Texas oilmen. Or by various Democrats, not least of them the man who was elevated to the presidency, Lyndon B. Johnson. One hypothesis held that the real target was John B. Connally, the Texas governor who was riding in the car with Kennedy.

During the 2016 campaign, Mr. Trump chimed in with a characteristically vague and unsubstantiated suggestion that Senator Ted Cruz’s father, a Cuban immigrant, had a connection to the killing.

The Kennedy assassination was a watershed moment, when public faith in government and other pillars of American civic life began to come undone. The unraveling is well underway, with conspiracy theories in full bloom.

Retro Report examines a particularly incendiary one of recent vintage: the outlandish assertion that Mrs. Clinton was behind a pedophile ring operating out of a Washington pizza parlor. Except for the existence of that pizzeria, nothing about this story bears a resemblance to reality. It should be laughable. Yet some take it seriously. One deluded man went to the parlor with an assault rifle so he could, as he put it, “self-investigate.” He was arrested and later pleaded guilty to weapons and assault charges.

Little has deterred American dealers in dietrologia, like Alex Jonesand Mike Cernovich. They and others vent paranoia on radio and in social media. And they have followers who are willing believers in shadowy intrigue: the Sept. 11 terrorist attacks were an inside job hatched by the Bush administration; the 2012 massacre at Sandy Hook Elementary School in Connecticut was a hoax. Last year a major candidate for a seat on Texas’ powerful State Board of Education drew national attention for having posted a smorgasbord of conspiracies on Facebook, including a claim that Mr. Obama was a male prostitute in his youth. (She lost the race.)

Trafficking in schemes is not uniquely American. Health workers seeking to vaccinate children against polio have been killed in Pakistan by militants who detect an anti-Muslim plot. The sudden death of Pope John Paul I in 1978, a mere 33 days into his reign, spurred conspiracy theories that endure. In some Arab societies, it is standard procedure to blame the Mossad, Israel’s intelligence service, for pretty much every calamity.

Still, the phenomenon has flowered in the United States. Death is notably fertile ground for conspiracy whisperers, like those who insist the Clintons killed their friend Vincent Foster and those who hint at foul play in the death last year of Justice Antonin Scalia of the Supreme Court.

Suspicion is ever-lurking now. This is a far cry from 1963 America. Opinion polls show a steep decline in public faith in institutions like the news media, academia, corporate boards, Wall Street banking houses and, for sure, the government. During his campaign, Mr. Trump capitalized on the distrust, and inflamed it, with ominous they’re-out-to-get-you warnings. The entire system is “rigged,” he said again and again.

Of course, modern tools of communication make it possible to spread falsehoods with lightning speed. A line often attributed to Mark Twain held that a lie can travel halfway around the world before the truth can get its boots on. With today’s technology, the lie can make it all the way to Neptune and back.

Twain himself had a dietrologia turn of mind. A line in his “Tom Sawyer’s Conspiracy” easily holds up amid present-day realities. What makes a successful scheme? Not truth or decency, that’s for sure. “Right hasn’t got anything to do with it,” Tom says. “The wronger a conspiracy is, the better it is.”

CLYDE HABERMAN, a regular contributor to Retro Report, has been a reporter, columnist and editorial writer for The New York Times, where he spent nearly 13 years based in Tokyo, Rome and Jerusalem. Subscribe to our newsletter here and follow us on Twitter @RetroReport.

This article first appeared in The New York Times.