Investigators have focused heavily on the question of whether Mr. Stone, who served briefly as a Trump campaign adviser and maintained contact with top campaign officials, knew where WikiLeaks got the stolen emails and how it planned to use them. Mr. Stone has said he was only boasting during the campaign when he claimed to be in touch with Julian Assange, the founder of WikiLeaks, about the purloined documents.

Mr. Corsi has publicly defended Mr. Stone, insisting that he was the source of a suspicious message that Mr. Stone posted on Twitter in August 2016, predicting that it would soon be Mr. Podesta’s “time in the barrel.” Mr. Stone posted the tweet before WikiLeaks began releasing tens of thousands of Mr. Podesta’s emails, throwing Mrs. Clinton’s campaign on the defense only one month before Election Day.

Mr. Corsi said that Mr. Stone was relying on his research into Mr. Podesta’s involvement in an offshore company that he thought cast the campaign official in a negative light. “I am confident that I am the source behind Stone’s tweet,” he said in one article posted on Infowars, a site that promotes conspiracy theories.

In his YouTube video, Mr. Corsi said that he had been interviewed repeatedly about WikiLeaks by a team of three prosecutors and multiple F.B.I. agents and that he had cooperated fully. He also testified before a federal grand jury in September in Washington.

Mr. Corsi said that if he had any inkling of what WikiLeaks had in store for the Clinton campaign, it was only because he knew how to use public information to “connect the dots.” But prosecutors confronted him with a thick binder of all of his communications, he said, and after repeated interrogations, “my mind was mush.”