WASHINGTON -- U.S. President Donald Trump changed his story Saturday on why he fired Michael Flynn as his national security adviser, now suggesting he knew at the time that Flynn had lied to the FBI as well as to Vice-President Mike Pence about his contacts with Russians during the presidential transition.

That was a turnabout from his initial explanations that Flynn had to go because he hadn't been straight with Pence about those contacts. Lying to the FBI is a crime, and one that Flynn acknowledged Friday in pleading guilty and agreeing to co-operate with the special counsel's Russia investigation.

Trump's "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!"

Amid questions raised by the tweet, Trump associates tried to put distance Saturday evening between the president himself and the tweet. One person familiar with the situation said the tweet was actually crafted by John Dowd, one of the president's personal attorneys. Dowd declined to comment when reached by The Associated Press on Saturday night.

In another email wrinkle in the investigation into Trump's ties to Russia, The New York Times reported Saturday that emails among top Trump transition officials suggested that Flynn was in close contact with other senior members of the transition team before and after he spoke to Russian Ambassador Sergey Kislyak. A Dec. 29 email from K.T. McFarland, a transition adviser to Trump, suggested that Russian sanctions announced by the Obama administration had been aimed at discrediting Trump's victory.

She wrote the sanctions could also make it more difficult for Trump to ease tensions with Russia, "which has just thrown the U.S.A. election to him," she wrote in the emails obtained by the Times. A White House attorney told the newspaper McFarland only meant that Democrats were portraying it that way.

It's unclear why Trump would cite lying to the FBI as a reason for firing Flynn. Doing so suggests the president knew at the time that Flynn had done something that is against the law, and therefore the investigation could not be as frivolous as he's been portraying.

It's also unclear how he would know that, if information about Russian contacts had not reached him, as he has been implying in his own defence.

Flynn left the White House in February, only acknowledging that he had given an incomplete account to Pence of his conversations with Kislyak. After Trump forced Flynn out, he asked FBI Director James Comey to end the bureau's probe in the matter, according to Comey's account. Comey refused, and Trump fired him, too.

Then-White House spokesman Sean Spicer said after Flynn's firing that it was the result of a "trust issue" and the White House counsel's office had determined there was not a legal issue.

"Whether or not he actually misled the vice-president was the issue, and that was ultimately what led to the president asking for and accepting the resignation of Gen. Flynn," Spicer told reporters on Feb. 14. "That's it, pure and simple. It was a matter of trust."

Trump has been publicly dismissive of Comey and of special counsel Robert Mueller's continuing investigation, and was often generous in his appraisal of Flynn, except to say his adviser could not stay on the job after misleading his vice-president.

At the time, Pence said Trump was justified in firing Flynn because Flynn had lied to him. Neither Trump nor Pence indicated concern then that the FBI had not been told the true story.

Pence, who served as head of Trump's transition, has not publicly commented on Flynn's plea.

Trump turned to Twitter again later Saturday night, railing about why Flynn was prosecuted but his Democratic opponent Hillary Clinton, was not for her alleged misdeeds.

He "So General Flynn lies to the FBI and his life is destroyed, while Crooked Hillary Clinton, on that now famous FBI holiday "interrogation" with no swearing in and no recording, lies many times...and nothing happens to her? Rigged system, or just a double standard?"

He "Many people in our Country are asking what the "Justice" Department is going to do about the fact that totally Crooked Hillary, AFTER receiving a subpoena from the United States Congress, deleted and "acid washed" 33,000 Emails? No justice!"

Clinton was voluntarily interviewed for more than three hours at FBI headquarters in July 2016. She was not in custody during her interview and so, in accordance with standard FBI and Justice Department protocol, her interview would not have been recorded. A Justice Department policy on recorded interrogations applies to individuals who have been arrested and are in custody.

It is nonetheless a crime to lie to the FBI about any material fact in an investigation. Comey has said Clinton did not lie to the FBI during her interview.