It seems to me to be a stretch to imagine that those tax returns are going to inform you about whether illegal activities took place a decade later, and that if they directly looked into those tax returns it would hardly have arisen from the investigation into possible Russia-Trump connections in the election.Following the Russian ties of Truump’s campaign managers seems like a reasonable place to start.
Now, if while looking into Manafort's actions surrounding the election they found evidence of him trying to hide those past criminal actions, that would actually come from investigating in regards to the purpose of the special counsel.
I'm not in a position to know how it came to be, but, I'd like to.
Broad investigative nets are what I have reservations about.But as I said, that investigation casts a very wide net and a lot of people have been caught in it already.