Former White House chief strategist Stephen K. Bannon has been subpoenaed by both the special counsel and the House Intelligence Committee to testify on their investigations into Russian interference in the 2016 election. (Reuters)

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…