(CNN) President Donald Trump questioned the basis of special counsel Robert Mueller's Russia investigation in a tweet early Saturday morning, alleging that the probe started after former FBI Director James Comey "illegally leaked classified" information.

"James Comey illegally leaked classified documents to the press in order to generate a Special Council? Therefore, the Special Council was established based on an illegal act? Really, does everybody know what that means?" Trump wrote at shortly after midnight.

James Comey illegally leaked classified documents to the press in order to generate a Special Council? Therefore, the Special Council was established based on an illegal act? Really, does everybody know what that means? — Donald J. Trump (@realDonaldTrump) April 21, 2018

The President has called the special counsel's probe a "witch hunt" as Mueller investigates whether Trump campaign associates colluded with Russia and any instances of obstruction of justice in the process of the investigation.

After he was fired by Trump, Comey testified before the Senate last year that he provided memos to "a friend" -- Columbia Law School professor Daniel Richman -- to read to the New York Times in hopes it would prompt the appointment of a special counsel to lead the Russia investigation.

The Department of Justice's inspector general is reviewing how Comey handled the memos, including whether any classified information was improperly shared, sources familiar with the matter told CNN

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