The F.B.I. conducts two types of inquiries, criminal and counterintelligence investigations. Unlike criminal investigations, which are typically aimed at solving a crime and can result in arrests and convictions, counterintelligence inquiries are generally fact-finding missions to understand what a foreign power is doing and to stop any anti-American activity, like thefts of United States government secrets or covert efforts to influence policy. In most cases, the investigations are carried out quietly, sometimes for years. Often, they result in no arrests.
Mr. Trump had caught the attention of F.B.I. counterintelligence agents when he called on Russia during a campaign news conference in July 2016 to hack into the emails of his opponent, Hillary Clinton. Mr. Trump had refused to criticize Russia on the campaign trail, praising President Vladimir V. Putin. And investigators had watched with alarm as the Republican Party softened its convention platform on the Ukraine crisis in a way that seemed to benefit Russia.