The slow-burn story in Washington that everyone should have their eye on: Is impeachment possible? Will Special Counsel Robert Mueller’s investigation into the Trump campaign’s pre-election relationship with Russia end in a formal call to remove the President from office? Will the news media lose its mind in the process?

Well, as we saw with the formal indictments against high-profile former Trump campaign figures Paul Manafort, Rick Gates, and George Papadopolous, Mueller is making moves with his investigation. With these three figures on the hook, Mueller has put himself in a position of leverage to get even more information and access into Trump’s world, and get to the bottom of who knew what, and when, about Russian’s efforts to throw the 2016 election.

And this news represents another major milestone: Mueller has issued a subpoena to the Trump campaign, requesting Russia-related documents from as many as 12 people close to Trump. This is a big deal because it’s the first time Mueller has used this formal legal mechanism to demand information from the Trump world; by issuing a subpoena, as opposed to just an informal request, Mueller is either a) signaling that Trump’s folks haven’t been all that forthcoming so far, and/or b) reminding the campaign officials that if they don’t comply, they face serious legal consequences.

All in all: This is just another indication that Mueller thinks he’s on to something, and is prepared to use the means at his disposal to uncover it.

To get caught up on our coverage of Mueller’s Russia probe and all it means for Trump, check this out. And hang on to your hats: There’s a lot more to go here.