The special counsel who finished the investigation, Robert S. Mueller III, secured three dozen indictments and convictions of some top Trump advisers, and he produced a report that outlined Russia’s wide-ranging operations to help get Mr. Trump elected and the president’s efforts to impede the inquiry.
But law enforcement officials never fully investigated Mr. Trump’s own relationship with Russia, even though some career F.B.I. counterintelligence investigators thought his ties posed such a national security threat that they took the extraordinary step of opening an inquiry into them. Within days, the former deputy attorney general Rod J. Rosenstein curtailed the investigation without telling the bureau, all but ensuring it would go nowhere.
A bipartisan report by the Republican-led Senate Intelligence Committee released this month came the closest to an examination of the president’s links to Russia. Senators depicted extensive ties between Trump associates and Russia, identified a close associate of a former Trump campaign chairman as a Russian intelligence officer and outlined how allegations about Mr. Trump’s encounters with women during trips to Moscow could be used to compromise him. But the senators acknowledged they lacked access to the full picture, particularly any insight into Mr. Trump’s finances.
Mr. Rosenstein concluded the F.B.I. lacked sufficient reason to conduct an investigation into the president’s links to a foreign adversary. Mr. Rosenstein determined that the investigators were acting too hastily in response to the firing days earlier of James B. Comey as F.B.I. director, and he suspected that the acting bureau director who approved the opening of the inquiry, Andrew G. McCabe, had conflicts of interest.
Mr. Rosenstein never told Mr. McCabe about his decision, leaving the F.B.I. with the impression that the special counsel would take on the investigation into the president as part of his broader duties. Mr. McCabe said in an interview that had he known Mr. Mueller would not continue the inquiry, he would have had the F.B.I. perform it.
“We opened this case in May 2017 because we had information that indicated a national security threat might exist, specifically a counterintelligence threat involving the president and Russia,” Mr. McCabe said. “I expected that issue and issues related to it would be fully examined by the special counsel team. If a decision was made not to investigate those issues, I am surprised and disappointed. I was not aware of that.”