Former federal prosecutor Andrew McCarthy said the FBI launching an investigation into whether President Trump was working on behalf of Russia was merely the formalization of a probe that had already been going on for more than a year.

The New York Times reported last week that the bureau opened a probe on Trump due to his behavior shortly after he fired former FBI Director James Comey in May 2017.

The inquiry carried explosive implications. Counterintelligence investigators had to consider whether the president’s own actions constituted a possible threat to national security. Agents also sought to determine whether Mr. Trump was knowingly working for Russia or had unwittingly fallen under Moscow’s influence.

The investigation the F.B.I. opened into Mr. Trump also had a criminal aspect, which has long been publicly known: whether his firing of Mr. Comey constituted obstruction of justice.

Agents and senior F.B.I. officials had grown suspicious of Mr. Trump’s ties to Russia during the 2016 campaign but held off on opening an investigation into him, the people said, in part because they were uncertain how to proceed with an inquiry of such sensitivity and magnitude. But the president’s activities before and after Mr. Comey’s firing in May 2017, particularly two instances in which Mr. Trump tied the Comey dismissal to the Russia investigation, helped prompt the counterintelligence aspect of the inquiry, the people said.

On "Tucker Carlson Tonight" Monday, McCarthy said when the FBI opened the probe in May 2017, it was merely a "formal paper matter."

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"They opened an investigation specifically on Trump, but it was the same investigation that had been being conducted for a year," McCarthy said.

He noted that the main theory of the infamous anti-Trump dossier -- which was used by the FBI to launch the initial Trump-Russia investigation and secure surveillance warrants to spy on a Trump campaign associate -- is that Trump personally conspired with Moscow to subvert the 2016 presidential election, after which Trump would do Russia's bidding from the White House.

"That was what they went to the FISA court with in order to get a warrant on Carter Page," McCarthy said. "The theory behind this -- if you look at the Steele dossier -- guys like Page and Manafort and Cohen, they're sort of the emissaries between the Trump side of the conspiracy and the Kremlin side of the conspiracy."

"But the whole thing's about Trump. And it was always about Trump. And all that happened in May of 2017 is they went overt in their files on something that they had been covertly doing for over a year."

In his monologue earlier in the show, Carlson stressed that the investigation was "unprecedented" in American history.

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"Shocking doesn’t even begin to describe this story. Nothing like this has ever happened in this country. The allegations are literally without precedent. That is not an overstatement," he said. 

He said Trump was raising fair questions during the 2016 campaign about the U.S.-Russia relationship, but "in Washington, saying that out loud was considered treason."

Carlson called for "radical transparency" on how the FBI and senior Justice Department officials launched the investigation and conducted it. 

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