【Watch The Grey Online】

Even for Donald Trump,Watch The Grey Online this tweet was a doozy.

On Friday, Trump's former national security adviser pled guilty to lying to the FBI. That alone is a big deal, but the real bombshell was that Michael Flynn had agreed as part of a plea deal to work with the FBI in its investigation into Russia's attempts to swing the 2016 U.S. election.

The next day, a tweet appeared about the news:

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If Flynn's plea was the shot, Trump's tweet was the chaser. It indicated that the president knew that Flynn had lied to the FBI — a serious crime — when he pushed then-FBI director James Comey to drop the investigation into Flynn. Trump fired Flynn in February, claiming that Flynn had lied to Vice President Mike Pence, but the FBI wrinkle wasn't part of the picture.

Trump then fired Comey in May, a move that immediately sparked accusations that Trump had committed obstruction of justice — a charge that both Richard Nixon and Bill Clinton faced as part of their impeachment hearings. By firing Comey, many reasoned, Trump had tried to scuttle the FBI investigation into Flynn. After the Comey firing, Robert Mueller was named special counsel to oversee the ongoing investigation.

Trump and his lawyers have, of course, denied that the president did anything to interfere with the investigation. Saturday's tweet, however, provided an important bit of evidence. If Trump knew Flynn had broken the law, it becomes even harder to believe Trump was not purposefully interfering in the investigation — particularly as Comey has said the president referenced Flynn by name in pushing him to end the Russia inquiry.

Trump's camp has been doing damage control on this. On Sunday, his personal lawyer John Dowd claimed that he wrote the tweet and that someone other than Trump posted it. The move appeared to be an effort to distance the tweet from Trump, casting doubt on whether the president knew Flynn had broken the law when he pushed Comey to end the Russia investigation.

This has led to questions about whether the tweet really could be used in a case against Trump. Here are the key questions with some expert insight:

Can tweets be used in legal proceedings?

The answer here is a clear yes.

Tweets have already been used in plenty of cases, including against Trump — particularly in relation to his attempts at issuing travel bans on certain Muslim-majority countries.

Even deleted tweets have been used in court cases. With Twitter, the main challenge is to show that the account belongs to a particular person, according to Alex Whiting, a law professor at Harvard University and a former prosecutor at the International Criminal Court. That's not an issue when it comes to Trump.

What is, though, is figuring out if what Trump says on Twitter, in a public space, matches what he says in private. Noah Feldman, also a Harvard professor, wrote on Bloomberg that the tweet alone probably wouldn't be enough to prove in an impeachment trial that Trump knew of Flynn's lies to the FBI.

"What’s needed to prove what Trump actually knew and the basis for his motivations when firing Flynn can’t be gleaned from this public format. To prove a crime, including a high crime, we need access to the alleged criminal’s private thoughts at the time of action. That can come in the form of testimony from confidants, contemporaneous emails or messages, or even inference from conduct. A tweet doesn’t qualify," Feldman wrote.

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The tweet is certainly admissible, but Trump's lawyers could try to claim it was just an erroneous thought. Plus, as Whiting noted, what really matters is whether a jury finds tweets significant, in general.

"Maybe it’s easier to explain away because Twitter, but it would be for the jury to decide. There is no per se rule that Twitter statements cannot be used to prove intent in a criminal case," Whiting wrote in an email.

Does it matter if Trump didn't draft or send the tweet?

It does matter if Trump had any hand in sending the tweet, but it might also not matter — especially since Trump has not come out and denied the tweet.

Let's run through a few scenarios here as posited by Whiting.

• Trump wrote and sent the tweet: The tweet would clearly then be attributed to Trump.

• Trump's lawyer wrote the tweet, Trump approved it: Trump is still at fault here. "If his lawyer wrote it, then Trump tweeted it, that's equivalent to him saying it. There's no difference," Whiting said.

• Trump's lawyer wrote the tweet, someone else tweeted it, Trump never saw it: This does give Trump some cover, but Whiting noted that since Trump has not denied that he knew Flynn had lied to the FBI and the tweet is still up, that could constitute him "adopting" the statement. "If he doesn't delete it from his Twitter account or correct it, then it could be argued that he has adopted it that way. By his actions and silence, he's adopted it," Whiting said.

One reason Trump may have not pushed back against the tweet is that it's true. Both the Washington Post and CNN have reported that Trump's White House lawyer told the president in January that he believed Flynn had lied to the FBI.

What if Trump's lawyer is lying?

If Trump's lawyer is lying, Mueller may well be able to find out. There's already rampant speculation that the special prosecutor will subpoena Dowd to get to the bottom of the tweet.

That gives Mueller another opportunity to either push for crucial information about what the president knows, or catch another Trump associate in a lie.

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Is Trump even worried about this?

It's hard to know what exactly is on Trump's mind at any particular moment, but it looked clear on Monday morning that the president has been worried about the obstruction argument.

Dowd, the lawyer who said he drafted the tweet in question, told Axios that a president can't obstruct justice "because he is the chief law enforcement officer ... and has every right to express his view of any case."

That point wasn't taken very seriously by legal analysts other than as an indication that Saturday's tweet, and the ensuing fallout, are weighing on the president's mind.