In 2013, Mr. Page struck up a professional friendship with the operative, Victor Podobny, who was working undercover in New York City. Mr. Page -- who at the time did not have any role in American government -- gave documents to Mr. Podobny about the energy sector.

Mr. Podobny was picked up by the authorities on a tapped phone calling Mr. Page an "idiot" to his Russian intelligence colleagues. He was charged by the Justice Department and spirited back to Moscow before he could be arrested. Mr. Page was questioned by law enforcement officials about his contacts but never charged in the case.

Mr. Page has openly acknowledged he is the unnamed male referred to in federal court documents about Mr. Podobny.

A dossier compiled by Christopher Steele, a former British intelligence operative hired to investigate Mr. Trump's links to Russia, claimed that Mr. Page maintained deep ties to the Kremlin, including with officials sanctioned by the United States.

Mr. Nunes's memo claims that the dossier, whose research was funded in part by Democrats, was improperly used to justify surveilling Mr. Page after he had cut ties with Mr. Trump. But the memo left out that the research was initially funded by The Washington Free Beacon, a conservative website.

Mr. Page's mannerisms amid the unfolding Russia investigations have emerged as a sort of Washington sideshow. Despite being under scrutiny by federal and congressional investigators as a possible agent of Russia, Mr. Page has waged a near-constant public affairs campaign, appearing on TV news shows and engaging with reporters, sometimes with obscure GIFs or movie clips.

For months, Mr. Page showed up regularly, uninvited and unannounced, at the secure offices of the Senate Intelligence Committee on Capitol Hill, where he dropped off documents he had compiled himself. One was his own dossier in which he claimed he was the victim of a hate crime by the Hillary Clinton campaign because he was a Catholic and a man.