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(CNN) —  

Former acting FBI Director Andrew McCabe said Thursday that a recent whistleblower complaint about a communication between President Donald Trump and a foreign leader is “incredibly concerning.”

“I think you are right. This President has a history of treating classified information and sensitive information questionably at best,” McCabe, a CNN contributor, told CNN’s John Berman on “New Day.”

The concern was first revealed by The Washington Post, which reported that an official in the American intelligence community was so bothered by a “promise” Trump made to a foreign leader that the official filed a formal whistleblower complaint with the inspector general of the intelligence community, citing two former US officials familiar with the matter. CNN has not independently confirmed the detail about a “promise” made to the foreign leader.

The revelation has opened fresh concerns about Trump’s dealings with foreign leaders, though the matter is shrouded in mystery as its details remain under wraps. McCabe, who also served as deputy director of the FBI, appeared baffled by the complaint Thursday, telling Berman that “it’s unfortunate that we’re left to kind of piece together … the puzzle.”

“But the fact that we now know it was the President and we know it involved some sort of a promise to a (foreign) leader is deeply concerning,” McCabe said.

A source familiar with the case who spoke to CNN would not disclose the foreign leader with whom Trump was reported to have spoken. The Office of the Director of National Intelligence declined to comment, and the White House has not responded to CNN’s request for comment.

The Post said its report was the first time Trump’s direct involvement in the complaint, which was filed August 12, has been reported. White House records show Trump had spoken to or interacted with five foreign leaders in the previous five weeks, the Post reports: Russian President Vladimir Putin, North Korean dictator Kim Jong Un, the Prime Minister of Pakistan, the Prime Minister of the Netherlands and the Emir of Qatar. However, it’s not clear the communication that inspired the complaint was with any of those leaders.

Trump’s handling of sensitive national intelligence has previously been scrutinized. CNN reported earlier this month that in 2017, the US extracted from Russia one of its highest-level covert sources inside the Russian government in part due to concerns that Trump and his administration repeatedly mishandled classified intelligence and could contribute to exposing the covert source as a spy.

CNN’s Zachary Cohen contributed to this report.