Former FBI Director James Comey is speculating that Special Counsel Robert Mueller could be in the “fourth quarter” of his Russia probe, citing the plea deal struck with former Trump campaign chairman Paul Manafort.

Comey, when asked at what stage he believes the Russia investigation is in, told St. Louis Public Radio 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,” Comey said.

Comey, fired by President Trump in May 2017, led the FBI when the bureau began investigating whether any people associated with the Trump campaign assisted the Russians in their effort to meddle in the 2016 election.

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That investigation eventually led to the appointment of Mueller as special counsel. The probe also led to charges against Manafort that were unrelated to the 2016 campaign.

Last month, Manafort was convicted on eight counts of bank and tax fraud, making him the first campaign associate of Trump found guilty in Mueller’s probe. Last week, Manafort pleaded guilty to charges in a separate case of foreign lobbying violations in a plea agreement that involves cooperation with Mueller.

Comey, who has come under criticism from President Trump and other Republicans over the Russia probe, stressed that he can’t say with “certainty” if Mueller is nearing the end of the investigation, which also includes looking at whether the president acted improperly through his firing of Comey.

“The reason I'm hesitant to even say that, is Bob Mueller 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,” Comey said. “And so I can't say with certainty where he is.”

Comey’s comments come the same week Trump ordered the declassification of several key documents related to the FBI investigation of Russian actions during the 2016 presidential election, including release text messages from Comey and a number of other key players in the Russia investigation "without redaction" -- including Bruce Ohr, Peter Strzok, Lisa Page and former FBI Deputy Director Andrew McCabe.

Fox News’ Samuel Chamberlain contributed to this report.