But he continued accumulating evidence, which emboldened Mr. Alford to put Mr. Ulbricht’s name on the D.E.A. database of potential suspects, next to the aliases altoid and Dread Pirate Roberts.

At the same time, though, Mr. Alford realized that he was not being told by the prosecutors about other significant developments in the case — a reminder, to Mr. Alford, of the lower status that the I.R.S. had in the eyes of other agencies. And when Mr. Alford tried to get more resources to track down Mr. Ulbricht, he wasn’t able to get the surveillance and the subpoenas he wanted.

Mr. Alford said the Manhattan federal prosecutor overseeing the investigation, Serrin Turner, seemed to want to find Dread Pirate Roberts more than anyone. But Mr. Alford said that Mr. Turner was working with multiple agencies on the case and did not seem to put much weight in the evidence that Mr. Alford was finding — leading to heated conversations.

“I’m not saying I’m right; we just need to look into this guy fully,” Mr. Alford remembers telling Mr. Turner.

A spokesman for the United States attorney’s office in Manhattan, where Mr. Turner works, declined to comment.

When Mr. Alford visited the main F.B.I. team on the case, later in the summer, it became clear that the team wasn’t aware of Mr. Ulbricht as a suspect — and also had no serious candidates of their own. Mr. Alford mentioned that he had a suspect in San Francisco, but no one followed up.

One of the other agents present for that meeting, who spoke on the condition of anonymity, said that he and the others in the room had little reason to ask for further information from Mr. Alford, given the lack of progress made by the D.E.A. Strike Force to which he was assigned. “No one was taking them seriously,” the agent said. “I obviously wished we had asked more.”