Special counsel Robert Mueller is investigating negative tweets posted by President Donald Trump about U.S. Attorney General Jeff Sessions and former FBI Director James Comey to determine whether the president obstructed justice during the ongoing probe into Russian election meddling, The New York Times reported Thursday.

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Mueller is examining the tweets to determine whether the president intimidated witnesses and pressured top law enforcement officers to drop the investigation, the Times reported, citing three unidentified sources familiar with the matter.

Trump's attorneys told the Times that "none of what Mr. Mueller has homed in on constitutes obstruction."

"If you're going to obstruct justice, you do it quietly and secretly, not in public," Trump attorney Rudy Giuliani told the Times.

The president has been publicly critical of both Sessions and Comey.

He referred to Sessions as “beleaguered” in a tweet last year in which he questioned the attorney general’s investigative priorities.

Reports suggest Trump has considered dismissing Sessions several times due to his frustration over Sessions’ decision to recuse himself in the investigation into Russian election meddling.

Comey was fired in May 2017, four years into his tenure as FBI director. In testimony before the Senate Intelligence Committee last year, Comey said he felt that Trump tried to pressure him to drop the investigation into former White House national security adviser Michael Flynn.

Flynn resigned less than a month into his tenure after reports surfaced that he lied to Vice President Mike Pence about his communications with Russian officials.

"I hope you can see your way clear to letting this go, to letting Flynn go," Trump told Comey one day after Flynn’s resignation in February 2017, according to a memo written by Comey. "He is a good guy. I hope you can let this go."

The president has denied several times that the Russia investigation played a role in his decision to dismiss Comey. However, he told NBC News in 2017 that he was thinking about "this Russia thing" when he came to his decision.

Mueller was tasked in May 2017 with investigating Russian election interference and its possible ties to Trump and his campaign officials. The investigation has led to charges against 32 people accused of crimes ranging from money laundering and falsifying income tax returns to lying to FBI investigators.

Five people, including Flynn, have pleaded guilty to charges in the investigation. The investigation is ongoing.