Donald Trump may have just dropped himself in it in a big way.

In a tweet he's admitted that he sacked Michael Flynn because he lied to the FBI over the nature of his conversations with the Russian ambassador.

It was AFTER the national security adviser was sacked that Mr Trump (allegedly) asked then-FBI director James Comey to drop the Flynn investigation, saying "he's a good guy".

He also subsequently tweeted that Mr Flynn was the victim of a witch-hunt and should seek immunity from prosecution.

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Yet, by the President's own admission on Sunday, he knew all along that Mr Flynn was guilty of a federal offence.

On Friday it was announced that Mr Flynn had cut a plea deal with FBI special counsel Bob Mueller, would plead guilty to conversations with the Russian ambassador that aimed to influence Russian foreign policy, and would assist the investigation.

Since Mr Trump fired FBI chief Mr Comey, part of that probe has been assessing whether the President aimed to obstruct justice by pressuring the FBI boss to drop the investigation into Mr Flynn, among other things.

Proven obstruction of justice could lead to impeachment.

Mr Mueller was already looking at Mr Trump's sacking of Mr Comey and whether that was part of an attempt to shut down the Russia investigation.

The President has now admitted that he knew Mr Flynn had committed a federal offence, yet at the very least encouraged him to seek immunity anyway, and at worst tried to get him off the hook by asking the head of the FBI to abandon the investigation into his activities.

Mr Flynn has said the decision to plead guilty was in the best interests of his family and of the country. ( Reuters: Jonathan Ernst )

Of the four members of Mr Trump's campaign team charged so far, Mr Flynn is also by far the closest to the President, and was very much part of the inner circle in the lead-up to the election.

He has now rolled over, and will reveal everything he knows to the special counsel.

But what Mr Trump says, or tweets, may be more telling.

Sunday's tweet could prove a turning point for the Trump administration, and not in a good way.

Timeline of key moments