On "America's Newsroom" Monday morning, former federal prosecutor and Fox News contributor Andrew McCarthy wondered how long Special Counsel Robert Mueller has known there was no collusion between the Trump campaign and Russia.

Attorney General William Barr on Sunday released the "principal conclusions" of Mueller's completed Russia probe in a letter to lawmakers, which stated definitively that Mueller did not establish evidence that Trump's team or any associates of the Trump campaign had conspired with Russia to sway the 2016 election.

McCarthy said that "there was never a collusion case here," but he argued there are still lingering questions about how the infamous anti-Trump dossier was compiled and what role it played in the Russia investigation.

He called the dossier a "Clinton campaign opposition research product," and claimed that other than that unverified document, there was never any evidence of the Trump campaign colluding with Russia.

McCarthy said that Mueller's team used the dossier to justify "moving the goalposts" away from the original point of the investigation, which was Russia's election meddling.

"What I wonder is how long Mueller's known that there's no collusion," he said.

He noted that when former FBI Director James Comey briefed Trump on the dossier in January 2017, Comey only gave the president a "small snapshot" of the dossier and did not mention that it alleged a massive Trump-Russia election conspiracy.

He added that Comey also did not mention that the dossier was used to obtain surveillance warrants on Trump campaign associates.

"They continued even after Trump was in office -- January, April, June -- to go back to the FISA court on the basis of this same information," McCarthy said. "Why didn't the FBI and the Justice Department try to verify it and corroborate it? And when they couldn't verify it, why didn't they refrain from going to the FISA court in the first place?"

See more from McCarthy above, and watch Trump attorney Rudy Giuliani react to Mueller's findings below.

Huckabee to Schiff: Show Your 'Evidence' of Collusion or 'Shut the Heck Up'

Dershowitz: Mueller Created a 'Comey Situation' by Punting on Obstruction Allegations

Concha on Mueller Probe Fallout: 'Day of Reckoning' for the Media