Mr. Hogg, the high school’s student news director, has become a sensation among many liberals for his polished and compelling television interviews, in which he has called on lawmakers to enact tougher restrictions on guns. Just as quickly, Mr. Hogg attracted the disdain of right-wing provocateurs like The Gateway Pundit, a fringe website that gained prominence in 2016 for pushing conspiracies about voter fraud and Hillary Clinton.

In written posts and YouTube videos — one of which had more than 100,000 views as of Tuesday night — Gateway Pundit has argued that Mr. Hogg had been coached on what to say during his interviews. The notion that Mr. Hogg is merely protecting his father dovetails with a broader right-wing trope, that liberal forces in the F.B.I. are trying to undermine President Trump and his pro-Second Amendment supporters.

Others offered more sweeping condemnations. Alex Jones, the conspiracy theorist behind the site Infowars, suggested that the mass shooting was a “false flag” orchestrated by anti-gun groups. Mr. Limbaugh, on his radio program, said of the student activists on Monday: “Everything they’re doing is right out of the Democrat Party’s various playbooks. It has the same enemies: the N.R.A. and guns.”

By Tuesday, that argument had migrated to CNN. In an on-air appearance, Jack Kingston, a former United States representative from Georgia and a regular CNN commentator, asked, “Do we really think — and I say this sincerely — do we really think 17-year-olds on their own are going to plan a nationwide rally?” (He was quickly rebuked by the anchor Alyson Camerota.)

Conspiracies, wild and raw online, are often pasteurized on their way into the mainstream. A subtler version of the theory appeared Tuesday on the website of Bill O’Reilly, the ousted Fox News host. Mr. O’Reilly stopped short of saying the students had been planted by anti-Trump forces. But, he wrote: “The national press believes it is their job to destroy the Trump administration by any means necessary. So if the media has to use kids to do that, they’ll use kids.”