Mr. Kaczynski ended up leaving school for journalism. At BuzzFeed he earned a reputation as a gadfly whose research methods (which he keeps secret) went far beyond those of the usual reporter with a Lexis-Nexis password.

In 2012 he provided proof that Mitt Romney had touted his Massachusetts health plan as a model for the Affordable Care Act. In 2013, he discovered that Senator Rand Paul of Kentucky had given a speech rife with passages from a Wikipedia entry on the movie “Stand and Deliver” and used erroneous quotes by founding fathers in two books.

Mr. Kaczynski has little in common with those glad-handing political reporters who work in the field as they court sources attached to this or that campaign. He is a digger who, along with his team, finds scoops back at headquarters.

Having received a diagnosis of pancreatitis at 19, Mr. Kaczynski neither smokes nor drinks. To keep up his health, he runs from five to six miles each morning while wearing a 12-pound training vest. Even then he is doing his job, listening to the hours of old audio he has loaded onto his iPhone.

Andrew Morse, the executive vice president for editorial at CNN U.S. and the general manager of CNN Digital Worldwide, wooed the reporter and his three colleagues away from BuzzFeed. He was especially impressed by the 2002 snippet, found by Mr. Kaczynski and Mr. McDermott, from the Howard Stern radio show episode in which Mr. Trump said he supported the Iraq war. (The candidate continues to deny he ever approved of the invasion, despite the recorded evidence to the contrary.)

“The light bulb really went off,” Mr. Morse said.

After accepting the job, Mr. Kaczynski changed his Twitter handle from @BuzzFeedAndrew to @KFILE. But the nature of his work did not change. Just before leaving BuzzFeed, he and another reporter found that Mr. Trump had appeared in a cameo role, fully clothed, in a soft-core Playboy video in 2000. In his first CNN scoop, Mr. Kaczynksi and another reporter unearthed a similar Trump appearance in a 1994 Playboy production.