Former CIA Director John Brennan complained late last year in an internal memo revealed Thursday that some lawmakers he had briefed on his agency's assessment that Russia meddled in the 2016 election did not "appreciate the importance and gravity" of the matter.

"In recent days, I have had several conversations with members of Congress, providing an update on the status of the review as well as the considerations that need to be taken into account as we proceed," he wrote in the December memo.

"Many — but unfortunately not all — members understand and appreciate the importance and gravity of the issue, and they are very supportive of the process that is underway."

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The memo was obtained by BuzzFeed News and Ryan Shapiro, a doctoral student at Massachusetts Institute of Technology. It was obtained as part of a Freedom of Information Act lawsuit against the CIA and other intelligence agencies.

Brennan, who left the CIA in January when President Trump took office, did not make clear which members of Congress he was referring to.

But BuzzFeed reported, citing three intelligences sources, that the former CIA chief's complaint was directed at Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellGraham: GOP will confirm Trump's Supreme Court nominee before the election Trump puts Supreme Court fight at center of Ohio rally The Memo: Dems face balancing act on SCOTUS fight MORE (R-Ky.) and Senate Majority Whip John Cornyn John CornynTumultuous court battle upends fight for Senate Texas Democrats roll out first wave of planned digital ads as Election Day nears Calls grow for Biden to expand election map in final sprint MORE (R-Texas).

The U.S. intelligence community released a report in January concluding that Russian officials sought to operate a hacking and influence campaign to swing the U.S. election in favor of then-candidate Trump.

That assessment prompted multiple congressional committees to launch their own investigations into Russia's role in the election, as well as possible collusion between the Trump campaign and Moscow.

Meanwhile, special counsel Robert Mueller is leading the federal investigation into the issue.

Trump has called the special counsel probe a "witch hunt," denying he did anything wrong or that his campaign coordinated with Russian officials in the presidential election.