In 2016, the F.B.I. sent an informant, a typical investigative step, to speak to two Trump campaign advisers after agents uncovered evidence that both had suspicious contacts linked to Russia during the campaign. When the role of the informant gained attention last year, the president seized on it to accuse law enforcement officials of illegally infiltrating his campaign.

Mr. Comey is familiar with the issue. In March 2017, Mr. Trump accused the F.B.I. and Obama administration in a tweet of illegally wiretapping Trump Tower during the campaign. The tweet angered Mr. Comey, who knew the claim was false and pushed the Justice Department to refute it. The Justice Department never released a statement, but Mr. Comey later said publicly that Mr. Trump’s claim had no merit.

“If the attorney general has come to the belief that that should be called spying, wow,” Mr. Comey said at a cybersecurity conference outside San Francisco. “That’s going to require a whole lot of conversations inside the Department of Justice. But I don’t know what he meant.”

Mr. Comey said that regardless of what the special counsel, Robert S. Mueller III, found during his investigation into Mr. Trump and his campaign, Mr. Barr already revealed, in his letter to Congress last month, that the investigation had uncovered important facts about Russia’s interference in the 2016 election.

“It tells us even without even reading the Mueller report that the Russia thing was not a hoax,” Mr. Comey said. “That it was real and that it is backed — that assessment is backed by hard evidence. It is true that the Russians came after us. They are going to come again because they exceeded their wildest hopes.”

Despite Mr. Barr’s claims about spying, Mr. Comey said he would give Mr. Barr the benefit of the doubt that he would put following the facts and law above protecting the president.

“Maybe the only thing I can say generally is I think his career has earned him a presumption that he will be one of the rare Trump cabinet members who will stand up for things like truth and facts and institutional values. So I still think he’s entitled to that presumption,” Mr. Comey said. “Language like this makes it harder, but I still think he’s entitled to that presumption and because I don’t understand what the heck he’s talking about, that’s all I can say.”