Random Human
They were trying to grab your prize. They work for the mercenary. The masked man.
I always felt like the tax returns didn't necessarily contain anything illegal but more embarrassing and that's why he never released them. The IRS has always had his tax info and could see if there was something illegal in there I would think? Unless the illegality of a situation is only clear with another piece of information that Mueller has dug up.
This is probably less about the specific tax returns and more that these guys are the ultimate "follow the money" people.