Historically, “false flag” conspiracy theories — named for a naval maneuver in which a ship flies a different country’s flag in order to trick enemies into retreating or to facilitate an escape — have remained on the edges of American discourse. Alex Jones, the conspiracy-theory-loving Infowars founder, was labeled a crank and worse for theorizing that the attacks of Sept. 11, 2001, were an “inside job,” and suggesting that the 2012 mass shooting at Sandy Hook Elementary School was an elaborate hoax concocted in order to promote gun control.

Mr. Jones has been largely pushed to the fringes of the internet — kicked off Twitter, Facebook and a dozen other services — and his cries for attention now seem mostly pitiful. (This week, he was filmed yelling at a pile of manure outside a rally for President Trump in Texas.)

But his spirit lives on in the larger universe of pro-Trump media, which has fused the conspiratorial grandeur of Infowars with an unshakable faith in Mr. Trump’s righteousness. Conspiracy theorists who might once have resorted to handing out subway pamphlets and shouting from street corners have found hungry, durable audiences on cable news shows and social networks. And false flag philosophy — the idea that powerful groups stage threats and tragic events to advance their agendas — is now a bizarrely common element of national news stories.

“The reason we’re seeing more false flag narratives is not that there are necessarily more of them, but that they’re more visible,” Ms. Merlan said. “It’s much easier for a casual news consumer to see them on Twitter.”

Conspiracy theories most often rise around fast-moving news events, like mass shootings and bomb threats, in which fuzzy initial reports often give way to more accurate explanations later on. And sometimes, those questioning the most apparent motives turn out to be justified.

Last year, a string of bomb threats against Jewish institutions in the United States was thought to be an act of anti-Semitic intimidation until police apprehended a Jewish teenager in Israel, whom they suspected of making the threats. The 2016 mass shooting at the Pulse nightclub in Orlando was thought to be an anti-L.G.B.T. hate crime until further evidence suggested the shooter had no idea it was a gay club.

One appeal of heat-of-the-moment conspiracy theories is that they allow for blame-shifting. Candace Owens, a right-wing activist and media personality who has been invited to the White House, earlier this week responded to the bombs by tweeting that “these leftists are going ALL OUT for midterms.” (Ms. Owens, who is now the communications director for the conservative student group Turning Point USA, appeared on Infowars with Mr. Jones as recently as last year.) She later deleted the tweet, but wrote another one in which she said that she still believed that “when it comes to political violence, the left is the likely culprit.”