If there's probable cause, I presume they could get a court order allowing search of NSA data archives. If that's not how it works, then legal wiretaps from the time of the court order going forward. Q has given the deep state lots and lots of opportunities to either confess and turn state's evidence, or display further criminal conduct.
Not a lawfag, just thinking about the copious evidence that likely exists, and the legal ways to obtain it.