Jesse Walker, the author of “The United States of Paranoia: A Conspiracy Theory,” said fear, the human need to find patterns and tell stories, and the recognition that conspiracies are not impossible help fuel such theories. The stories — no matter how outlandish — can bring meaning and a measure of comfort in a world that can make no sense, he said.

False-flag theories have long been around. One focused on the assassination attempt in 1835 of President Andrew Jackson, during which the president fought off a gunman whose two weapons misfired. Conspiracy theorists at the time believed Jackson had hired the gunman as a way to drum up sympathy for himself, Mr. Walker said.

Unlike the 1800s, stories today benefit from instant delivery through the internet and social media. One of the better-known purveyors is Alex Jones, who hosts an internet show at the website infowars.com. The day of the Orlando shooting, he posted a video in which he asserted that the government had let the massacre happen so it could pass “hate laws to deal with right-wingers” and to disarm gun owners. He did not respond to an email seeking comment .

Mike Rothschild of Pasadena, Calif., who has researched and written about conspiracy theories, described the world of false-flag believers as a “bank of awakened internet sleuths that has got it all figured out.” They see it as their duty to warn others about secret elites in government who are plotting against citizens, he said.

If overwhelming concrete evidence debunks the theorists’ notions, it only reinforces their ideas, said Chip Berlet, a researcher of radical-right movements and retired analyst at Political Research Associates, a left-leaning think tank in Somerville, Mass. For conspiracy believers, explaining it away “shows how smart the enemy really is,” he said.

Rob Brotherton, a psychologist and science journalist who wrote “Suspicious Minds: Why We Believe Conspiracy Theories,” said it is not just false-flaggers who seek connections and hidden meanings in world events.