They did this very thing to associates of Trump in the course of the Mueller investigation. Unrelated tax issues, failing to register as a foreign agent and other charges were brought against certain individuals for things they did years before they joined the Trump campaign. Whatever your thoughts are on this, there is already established precedent for it.
Which all seem like notable things to encounter when doing an investigation like that and to not drop when encountered?
'Let's look if money was sent from x to y. Get bank and tax statements. Well (it didn't come from x but) it sure as hell wasn't taxed'
It's not like this was brought up as part of a single court case if not related which would make it a relevant precedent here.
