(CNN) After a CNN report that special counsel Robert Mueller is looking into President Donald Trump and his associates' financial ties to Russia, crossing what Trump himself called a red line, James Clapper, the former director of national intelligence, warned Trump on Thursday night that firing Mueller could have damaging consequences for both Trump and the country.

Federal investigators exploring whether Trump's campaign colluded with Russian spies have seized on Trump and his associates' financial ties to Russia as one of the most fertile avenues for moving their probe forward, according to people familiar with the investigation. The web of financial ties could offer a more concrete path toward potential prosecution than the broader and murkier questions of collusion in the 2016 campaign, these sources said.

During an interview on CNN's "Erin Burnett Outfront," Clapper, who led the intelligence community's assessment on Russian interference in the election, questioned Trump's ability to establish any red line, contending, "I'm not sure how someone who is under potential investigation draws red lines and says that certain aspects are out of bounds, I don't know how that works."

But he said it would be "very dangerous" if this led to Trump firing or attempting to fire Mueller, warning that "I think this would create a real constitutional crisis. This would be Watergate in slow motion, which I also lived through."

"Good on General McMaster...he's done exactly the right thing" James Clapper on Susan Rice keeping sec. clearance https://t.co/3SAVIpM7VD — OutFrontCNN (@OutFrontCNN) August 3, 2017

Clapper's interview came as a person familiar with the matter confirmed to CNN that the special counsel has issued grand jury subpoenas related to the Donald Trump Jr. meeting at Trump Tower in June 2016, seeking both documents and testimony from people involved in the meeting.

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