On Saturday, the president stunned some legal observers when he said on Twitter that “I had to fire General Flynn because he lied to the Vice President and the FBI. He has pled guilty to those lies. It is a shame because his actions during the transition were lawful. There was nothing to hide!”

Susan Hennessey, a former attorney in the National Security Agency’s office of general counsel, noted in response that “this is a pretty substantial confession to essential knowledge elements of an obstruction of justice charge.” The tweet was the first confirmation that Trump himself knew that Flynn had lied to the FBI when he reportedly pressured Comey to back off prosecuting Flynn, meaning that the president could have been attempting to obstruct justice when he did so.

Two sources told the Washington Post that the tweet was drafted by the president’s attorney, John Dowd.

Trump fired Comey in May, and has said publicly that he was considering the Russia investigation when he did so; that fueled allegations of obstruction of justice. A week after Comey’s firing, Deputy Attorney General Rod Rosenstein appointed Robert Mueller as special counsel, granting him a broad mandate to investigate crimes related to Russian interference, including potential obstruction.

“If it turns out that General Flynn has information implicating Mr. Trump in a crime, there’s now a much stronger inference that Mr. Trump was obstructing justice if he asked Comey to let the investigation of General Flynn go,” said Bruce Green, a law professor at Fordham University and a former associate special counsel in the Iran-Contra affair.

According to the statement of the offense Mueller issued, Flynn informed a senior member of the Trump transition team on December 29, 2016, that Kislyak had contacted him. During that conversation, they discussed that senior members of the transition team did not want Russia “to escalate the situation” with regard to sanctions. On January 24, four days after Trump took office, Flynn attempted to mislead federal investigators looking into the matter. Acting Attorney General Sally Yates testified to Congress that she warned the White House that Flynn had been compromised by those conversations on January 26, four days before she was fired for ordering Justice Department officials not to defend Trump’s newly issued travel ban. Flynn was not forced to resign until February 13, after his conversations with Kislyak were reported in the press.

“Many points that might form the basis of an obstruction case flow through Flynn,” said Clinton Watts, a former FBI special agent who is currently a fellow at the Foreign Policy Research Institute. “President Obama warned Trump about Flynn before he took office. Yates contacted the White House about Flynn and was fired shortly after. Trump pressured Comey about Flynn and then later fired him.”