Former FBI Director James Comey James Brien ComeyDemocrats fear Russia interference could spoil bid to retake Senate Book: FBI sex crimes investigator helped trigger October 2016 public probe of Clinton emails Trump jabs at FBI director over testimony on Russia, antifa MORE said Wednesday he thinks special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE may be in "the fourth quarter" of his investigation into Russian meddling in the 2016 election.

Comey, in an interview with St. Louis Public Radio, pointed to former Trump campaign chairman Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE's agreement with prosecutors last week to cooperate with the probe as part of a guilty plea.

The former FBI chief said that there's “an argument to be made that the conviction — the plea and cooperation by Paul Manafort — may represent that we’re in the fourth quarter.”

“The way you normally do investigations is you work from the bottom up, and so they're getting pretty high,” he added.

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Comey cautioned, however, that he "can't say with certainty" where the investigation stands because Mueller has "conducted his investigation like a pro."

"The reason I'm hesitant to even say that is [because] Bob Mueller's conducted his investigation like a pro — you know nothing about it except through his public filings, and that's the way it's supposed to be. And so I can't say with certainty where he is.”

Trump fired Comey as FBI director last year, prompting the appointment of Mueller to investigate Russia's meddling in the 2016 election and any links between the Trump campaign and Moscow.

The president has relentlessly attacked the special counsel investigation as a "hoax" and a "witch hunt" and has even threatened to shut it down himself.

Trump has also criticized the Department of Justice as a whole, calling for an investigation into what he claims is bias against him, and has gone after Comey personally.

"I should have fired him before I got here," Trump said of Comey during an interview Tuesday with Hill.TV. "I should have fired him the day I won the primaries. I should have fired him right after the convention. Say, ‘I don’t want that guy.’ Or at least fired him the first day on the job."

Comey during his interview Wednesday criticized Trump's attacks on "the rule of law and the institutions of justice."

“I think we’re in two different places,” Comey said. “We’re in a place where the president of the United States relentlessly attacks the rule of law and the institutions of justice, so that’s terrible. But the second place that we’re in is that Americans have awakened to the importance of the rule of law and the danger of its erosion, and that’s a very, very important sort of antibody response. And it’s a source for optimism.”