(Alex Jones claimed the U.S. was behind the Boston bombings to strip Americans’ civil liberties.)

Boylston Street was literally slick with blood when Massachusetts Gov. Deval Patrick held a press conference on April 15. The first question came from Dan Bidondi. “Why were the loud speakers telling people in the audience to be calm moments before the bombs went off?” he demanded. “Is this another false flag staged attack to take our civil liberties and promote homeland security while sticking their hands down our pants on the streets?” Patrick dismissed him (“No, next question”), but a lot of listeners must have been mystified by the exchange.

Bidondi’s credentials are from Alex Jones’s Infowars. Within minutes of the blast, Jones had Tweeted that “this thing stinks to high heaven.” First he speculated that the bombing was an attempt to distract attention from the falling price of gold, but within a few hours the consensus at Infowars was that it was a “planned event to justify a TSA lockdown… the run-up to the TSA occupation of America, which has always been the goal of Obama.” And Infowars (which dead Boston bomber Tamerlan Tsarnaev purportedly followed closely) is just the tip of the conspiracist iceberg.

Though conspiracy theories began to emerge almost instantly after the JFK assassination—for example, the white supremacist Revilo Oliver’s notorious essay “Marxmanship in Dallas,” which argued that Kennedy was betrayed and murdered by his Communist overseers as a prelude to a Communist takeover—several years went by before they bubbled up into the mainstream. Thanks in large part to the Internet, the 9/11 Truth movement reached critical mass much faster. After the Newtown killings, it was just a matter of days before Sandy Hook Truthers rose to the fore. In Boston, the official and the alternative narratives were composed simultaneously.

If you’re a hammer, as the saying goes, everything looks like nails. The morning after the blasts, the lead post on the extreme anti-Semitic site Wake from Your Slumber was “Mossad Pulls off Boston Marathon Attacks.” Islamophobe Pamela Geller saw Arabs and Jihad. Larry Klayman, founder of Judicial Watch, also attributed the blasts to Muslims. “Neville Chamberlain of pre-World War II days would have been proud of Obama and his pro-Muslim minions,” he wrote in World Net Daily. Glenn Beck kept the focus on a Saudi student who was briefly questioned.

There are conspiracists on the left as well. James Tracy–the Florida Atlantic University professor who had garnered outraged national attention when he declared that the grieving families in Newtown were actors–posted “evidence” on his blog that purported to show that the explosions in Boston were a “mass-casualty drill” carried out by actors.

Tracy’s callousness is repugnant, but he has a point about taking media reports with a grain of salt. An “M-4 carbine rifle … [was found] on the boat where the younger suspect was found Friday night in Watertown, Mass.,” The New York Times reported after Dzokhar Tsarnaev was taken into custody. Unnamed sources told reporters that the brothers had used two handguns and a BB gun firing “roughly 80 rounds” during a shootout with police in which Tamerlan Tsarnaev was fatally wounded. The brothers, as it turned out, had just one gun between them. The wounded transit cop was very likely struck by friendly fire. Tamerlan Tsarnaev died from wounds suffered after Dzokhar ran him down with a car and Dzokhar Tsarnaev was unarmed at the time of his arrest.

But if journalists bear witness to the elusiveness of truth, the fantasies purveyed by conspiracy theorists are of an altogether different epistemological order. Conspiracy theory is essentially Platonic. Though its proponents claim to be skeptics, they hew to a transcendent set of certainties: In most cases, their first principle is that a shadowy but clearly identifiable group is working to take over the country.

The “paranoid style,” as Richard Hofstadter termed it, has been a feature of American politics since colonial times. For conspiracists on the right today, that shadowy but clearly identifiable group is as often as not identified as Islam. For the most part, conspiracy theory remains on the fringe. As hateful and intellectually lazy as it can be, it generally does little real harm. But when it does break into the mainstream, it can do considerable mischief.

As disturbing as it may be that a handful of Tea Party Congressmen are taking their cues from Glenn Beck, it’s even more troubling that opponents of immigration reform in the Senate are exploiting the resurgent Nativism stirred up by the Boston bombing to derail the immigration reform bill. More dangerous by far than sincerely loopy extremists are the politicians and pundits who co-opt their obsessions and use them opportunistically.

Arthur Goldwag is the author of The New Hate: A History of Fear and Loathing on the Populist Right. Follow him at @ArthurGoldwag and @WashSpec.