President Trump said Saturday he had to fire Michael Flynn as national security adviser because he lied to Vice President Pence and the FBI, an apparent shift from the reasons Trump stated at the time of Flynn's ouster.

Trump’s changing rationale is a discrepancy that will catch the attention of prosecutors on Mueller’s team looking into whether the president tried to obstruct the investigation.

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"I had to fire General Flynn because he lied to the Vice President and the FBI. He has pled guilty to those lies," Trump tweeted while traveling in New York City for fundraising events.

Trump also stressed “there was nothing to hide!”

"It is a shame because his actions during the transition were lawful. There was nothing to hide!" he added.

The president’s tweet is his most substantial reaction yet to Flynn’s guilty plea in special counsel Robert Mueller's Russia investigation.

Before leaving the White House on Saturday, Trump told reporters there was “no collusion” between his campaign and Moscow — but did not elaborate.

Flynn pleaded guilty on Friday to lying to federal investigators about his conversations during the presidential transition with Russia's U.S. ambassador about sanctions related to the Kremlin’s election interference.

The retired Army general said those topics did not come up in his talks with then-Ambassador Sergey Kislyak, a claim that was repeated publicly by Pence and other White House officials.

Trump in February said he axed Flynn because he lied to the vice president — but did not mention anything related to what the national security adviser told the FBI.

“I fired him because of what he said to Mike Pence Michael (Mike) Richard PenceGardner signals support for taking up Supreme Court nominee this year Biden leads Trump by 12 points among Catholic voters: poll GOP brushes back charges of hypocrisy in Supreme Court fight MORE. Very simple,” Trump said at a Feb. 16 news conference, three days after Flynn was fired.

In June, former FBI Director James Comey testified before Congress that Trump asked him to end the investigation into Flynn during an Oval Office meeting on Feb. 14. Trump's lawyers have disputed that account.

Trump fired Comey in May.

If Trump knew that Flynn had lied to the FBI and then asked Comey to drop the investigation, it could amount to obstruction of justice, according to some legal experts.

"This is a pretty substantial confession to essential knowledge elements of an obstruction of justice charge," tweeted Susan Hennessey, a national security fellow at the Brookings Institution.

But it is unclear if Trump’s tweet offered new details about what he knew at the time or if the president was using Flynn’s plea to retroactively explain his firing.

Either way, Flynn’s confession has dramatically raised the legal stakes for Trump and the White House in the Mueller probe.

The former national security adviser is the first person to have held a position in the Trump administration to be charged by the special counsel.

Flynn, who was a close Trump confidant during the 2016 election, said in a statement Friday he is cooperating with Mueller's team, meaning it is possible he could reveal damaging information about Trump or members of his inner circle.