WASHINGTON — Ever since F.B.I. investigators discovered in 2013 that a Russian spy was trying to recruit an American businessman named Carter Page, the bureau maintained an occasional interest in Mr. Page. So when he became a foreign policy adviser to the Trump campaign last year and gave a Russia-friendly speech at a prestigious Moscow institute, it soon caught the bureau’s attention.

That trip last July was a catalyst for the F.B.I. investigation into connections between Russia and President Trump’s campaign, according to current and former law enforcement and intelligence officials.

It is unclear exactly what about Mr. Page’s visit drew the F.B.I.’s interest: meetings he had during his three days in Moscow, intercepted communications of Russian officials speaking about him, or something else.

After Mr. Page, 45 — a Navy veteran and businessman who had lived in Moscow for three years — stepped down from the Trump campaign in September, the F.B.I. obtained a warrant from the Foreign Intelligence Surveillance Court allowing the authorities to monitor his communications on the suspicion that he was a Russian agent.