The State Department helped push a false narrative that led to the opening of the Trump-Russia investigation, according to former U.S. Attorney Andrew McCarthy.

In an opinion piece for National Review, McCarthy said a key takeaway from special counsel Robert Mueller's investigation relates to a conversation Trump campaign adviser George Papadopoulos had with an Australian diplomat that led to the FBI launching its counterintelligence investigation into the Trump campaign.

Mueller found Papadopoulos told the diplomat, Alexander Downer, the Russians had damaging information on Hillary Clinton, Trump's Democratic rival in the 2016 election. But it wasn't until two months later, in July 2016 when WikiLeaks published stolen emails from the Democratic National Committee, that Downer decided to reach out to the U.S. embassy in London about the conversation. This prompted the FBI's counterintelligence investigation, called Crossfire Hurricane, in July 2016.

But, as McCarthy noted, Papadopoulos never told Downer about emails, and Mueller's 448-page report "provides no basis" to show Papadopoulos knew the Russians planned to released damaging information about Clinton to boost Trump.

[Related: FBI may have used confidential sources prematurely: ex-official]

This resulted in the "unfounded inference that the hacked emails must have been what Papadopoulos was talking about" when they met in London in early May, McCarthy said. Despite it being a "flawed assumption," McCarthy said it was all too eagerly accepted by the Trump-opposed Obama administration, which had been toying around with a theory, thanks to unverified reports from British ex-spy and dossier author Christopher Steele, that the Kremlin was in the tank for Trump.

"Downer’s flawed assumption that Papadopoulos must have been referring to the hacked DNC emails was then inflated into a Trump–Russia conspiracy theory by Clinton partisans in the Obama administration — first at the State Department, and then in the Justice Department, the FBI, and the broader intelligence community — all agencies in which animus against Donald Trump ran deep," McCarthy wrote.

The man who informed Papadopoulos of this "dirt" was Maltese academic Joseph Mifsud, who has long been suspected of deep ties to Russian intelligence. Although Papadopoulos, who pleaded guilty to lying to the FBI, told investigators Mifsud talked to him about the Russians having "thousands" on Clinton emails, Mifsud denied to the FBI he knew or mentioned anything about emails and Mueller did not allege he had been untruthful.

[Also read: Devin Nunes: Clinton 'dirt' tipster Joseph Mifsud has ties to State Department]

McCarthy noted that Mueller never quoted Downer, instead "disingenuously" stressing the “suggestion” that Papadopoulos was referring to stolen emails. Furthermore, the emails to which Papadopoulos alluded were not the ones stolen from the DNC but rather emails that Clinton had stored on a unauthorized, private server during her time as secretary of state.

Thus, the State Department and FBI sought to "distort" what Papadopoulos told Downer.

"The State Department’s report to the FBI claiming that Papadopoulos had 'suggested' these things to Downer was manufactured to portray a false connection between (a) what Papadopoulos told Downer and (b) the hacking and publication of the DNC emails," McCarthy concluded. "That false connection then became the rationale for formally opening the FBI’s Trump-Russia investigation — paper cover for an investigation of the Trump campaign that was already under way."