There have been a few moments in the Russia investigation that, in retrospect, seemed to indicate a notable shift or an expansion. The day US President Donald Trump fired FBI director James Comey. The Lester Holt interview. When we first found out about the meeting with the Russian lawyer. The early-morning raid of Paul Manafort's Virginia residence. Michael Flynn's guilty plea.
Tuesday may have provided a smaller but significant one.
We found out that special counsel Robert Mueller's investigation last week issued its first known grand jury subpoena to a member of Trump's inner circle. The recipient: Steve Bannon.
The story, which was first reported by the 'New York Times', broke the same day that Mr Bannon was testifying behind closed doors to Congress, and it suggests that Mr Mueller is switching up tactics - or at least starting to employ new ones. The big, unanswered question is: why?
To this point, all indications are that members of Mr Trump's inner circle have spoken to investigators voluntarily and without being served grand jury subpoenas. Subpoenas are generally used to compel witnesses to give information that they may not otherwise volunteer. They can also be used to force witnesses to respond on the record in a more accountable setting to information they weren't previously aware of - without being able to lean on a lawyer.
So basically, if Mr Mueller has a juicy piece of inside info that he wants Mr Bannon to respond to, he may want that exchange to take place in front of the grand jury so that Mr Bannon feels pressured to respond more quickly and to be more forthcoming.
But even with all of that, the question is: why Mr Bannon? The former top campaign aide and White House adviser thus far hasn't been nearly as central to the Russia probe as other current and former members of Mr Trump's inner circle - people like Mr Flynn, Jared Kushner and Donald Trump Jr. Even Mr Manafort.
Mr Bannon wasn't in that Russian-lawyer meeting with Mr Manafort, Mr Kushner and Mr Trump Jr, for instance, and he wasn't present when the decision was made to fire Mr Comey.
Mr Bannon has also retained the same lawyer as former Trump chief of staff Reince Priebus and White House counsel Don McGahn, and both of them have talked to Mr Mueller's team without having subpoenas thrown their way.
Could it have anything to do with the Michael Wolff book, which quoted Mr Bannon labelling that meeting with a Russian lawyer as being "treasonous" and as believing Mr Mueller's probe was "all about money laundering?"
Mr Bannon reportedly told Wolff: "The Kushner s*** is greasy. They're going to go right through that." That just happens to be when the subpoena lands? It seems possible that Mr Mueller wants to send a signal to Mr Bannon, who seems anxious to reconcile with Mr Trump, that he won't make that easy on him. (Mr Bannon refused to answer certain questions in front of the House intelligence committee on Tuesday because the White House had cited executive privilege, and he earned himself another subpoena there for it. But yesterday it was reported that the former White House chief strategist has agreed to answer questions from prosecutors on the Russian election meddling investigation.)
Another intriguing possibility is that Mr Mueller wants to take advantage of Mr Bannon's tendency to spout off in real time and in an unforgiving setting. While plenty have focused on Mr Bannon's comments in Wolff's book last week, this isn't the first time he has publicly acknowledged serious errors by the Trump team with regards to Russia.
During a '60 Minutes' interview back in September, you might recall, Mr Bannon labelled Mr Comey's firing the single biggest mistake in modern political history. Perhaps Mr Mueller just wants to make sure Mr Bannon is afforded the opportunity to, well, be as candid as possible.
Nobody outside of Mr Mueller's team truly knows what the reason is. But when I think about this, I keep going back to that early-morning raid at Mr Manafort's house. It was seen at the time as an extraordinary and unprecedented measure - the kind of thing you do only when you worry that a witness might destroy evidence. And indeed, that's a requirement for getting a warrant to conduct such raids.
Fast-forward a few months, and the reason for the raid was clear: Mr Manafort has been indicted.
Exactly why Mr Mueller is switching up tactics here is really important. But the mere fact that he is doing so also seems to be.
And Mr Mueller issuing subpoenas to Mr Trump's closest advisers (even former ones) seems like a big moment in an investigation that hasn't wrapped up nearly as quickly as Mr Trump's team thought or hoped.
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