There have been a few moments in the Russia investigation that, in retrospect, seemed to indicate a notable shift or an expansion. The day President Trump fired FBI Director James B. 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 just found out that special counsel Robert S. Mueller III's investigation last week issued its first known grand jury subpoena to a member of Trump's inner circle. The recipient: Stephen K. Bannon. The story, which was first reported by the New York Times, broke the same day that Bannon was testifying behind closed doors to Congress, and it suggests that 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 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 Mueller has a juicy piece of inside info that he wants Bannon to respond to, he may want that exchange to take place in front of the grand jury so that Bannon feels pressured to respond more quickly and to be more forthcoming.

But even with all of that, the question is: Why 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 Trump's inner circle — people such as Flynn, Jared Kushner and Donald Trump Jr. Even Manafort. Bannon wasn't in that Russian-lawyer meeting with Manafort, Kushner and Trump Jr., for instance, and he wasn't present when the decision was made to fire Comey. 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 Mueller's team without having subpoenas thrown their way.

Could it have anything to do with the Michael Wolff book, which quoted Bannon labeling that meeting with a Russian lawyer as being “treasonous” and as believing Mueller's probe was “all about money laundering”? 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?

Stephen K. Bannon on March 15, 2017. (Nicholas Kamm/AFP/Getty Images)

It seems possible that Mueller wants to send a signal to Bannon, who seems anxious to reconcile with Trump, that he won't make that easy on him. (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 allies insist he's willing to cooperate with Mueller.)

Another intriguing possibility is that Mueller wants to take advantage of Bannon's tendency to spout off in real time and in an unforgiving setting. While plenty have focused on 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, Bannon labeled Comey's firing the single biggest mistake in modern political history. Perhaps Mueller just wants to make sure Bannon is afforded the opportunity to, well, be as candid as possible.

Nobody outside of 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 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: Manafort has been indicted.

Exactly why Mueller is switching tactics here is really important. But the mere fact that he is doing so also seems to be. And Mueller issuing subpoenas to Trump's closest advisers (even former ones) seems like a big moment in an investigation that hasn't wrapped up nearly as quickly as Trump's team thought or hoped.