The theory about how law enforcement investigates criminal organizations is that they start at the bottom to arrest low-level participants, getting them to agree to lesser charges in exchange for flipping on their superiors within the organization. That keeps going until the investigators reach the top of the food chain.
To go back to my comparison of the Mueller probe to the first season of The Wire, there is a classic scene where D’Angelo Barksdale explains the game of chess to his runners Wallace and Bodie. They immediately connect it to the structure of the Barksdale criminal organization.

I’ve been thinking about all of that the last few days as we try to absorb the latest events with Trump’s former campaign manager Paul Manafort and his former attorney Michael Cohen. To use D’Angelo’s chess comparison, it looks like the investigation has reached the “queens” of the conspiracy between the Trump campaign and Russia.
To the extent that the Steele dossier provided an accurate description of that conspiracy, here is what was reported in the early stages on page 7.

The “conspiracy of cooperation” was managed on the Trump side by Paul Manafort. Then on August 19, 2016, he resigned from his position as campaign manager due to revelations about the payments he received from Putin’s man in the Ukraine, former President Viktor F. Yanukovych. Who took his place as the manager of the conspiracy? Here’s what the dossier says about that on page 32.

That would be Michael Cohen, which is why he is alleged to have traveled to Prague to meet with Putin’s emissaries.
What we are watching at this point is how Robert Mueller is closing in on the two “get shit done” players in the conspiracy of cooperation between Trump and Putin. The special counsel has bagged a lot of pawns in this investigation and we don’t know how much of the big picture he’s been able to verify so far. But Mueller strikes me as someone who is relentlessly thorough. In order to build the complete case of how Russia attempted to influence the 2016 election and the role of the Trump campaign in that process, he wants to also bag the guys that managed the whole thing for the president. Flipping the queens is how he checkmates the king.