George Washington University law professor and legal expert Jonathan Turley said Monday that President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE's former personal attorney Michael Cohen flipping could mean that the president is "one witness away from a potential catastrophe."

“I think the Cohen development is very serious,” Turley, who is an opinion contributor at The Hill, said on MSNBC's "Morning Joe." “He’s one witness away from a potential catastrophe. If any of those five witnesses breaks and supports Michael Cohen, this will get real bad, real fast."

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Turley made the remarks just days after CNN reported that Cohen said Trump knew of a 2016 Trump Tower meeting between his son and a Kremlin-connected lawyer ahead of time.

Turley added that "it’s not that the meeting will establish a crime of collusion, even if what Cohen is saying is true, but what it would mean is that Donald Trump Jr. Don John Trump'Tiger King' star Joe Exotic requests pardon from Trump: 'Be my hero please' Zaid Jilani discusses Trump's move to cancel racial sensitivity training at federal agencies Trump International Hotel in Vancouver closes permanently MORE would be in serious jeopardy of a criminal charge."

“If Mueller was to go after Donald Trump Jr., I think we would see a very rapid chain of events and it would not end well for anyone," he said, adding that Trump could react to an event like this by firing multiple people, which could lead the country closer to potential impeachment.

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The comments come as Trump and his legal team push back hard against the claims that Cohen made in regards to his knowledge of an infamous Trump Tower meeting during the 2016 presidential campaign.

CNN reported that Cohen does not have evidence to back up his story, such as audio recordings, but that he is willing to discuss it with special counsel Robert Mueller. Mueller is leading an investigation into Russian interference in the most recent presidential election.

Trump's lead attorney in the Russia probe, Rudy Giuliani, said on Sunday that Cohen's claim is "flat-out untrue" while speaking on "Fox News Sunday."

Trump has also spoken out against Cohen since, and resurfaced an old tweet from him where he defended Trump Jr. for his transparency regarding the meeting.

"Do you think the Fake News Media will ever report on this tweet from Michael?" Trump said.