I know that I’m saying how did they link them to the cartel again? Sorry i binge it in a few days 1-2 years ago
I think they knew Marty's business partner was murdered by the cartel, then they see him suddenly move and start moving a lot of money around.
they had 18 wires in his business partner apartment alone, they show the feds removing all the bugs after they died and they wonder why Marty didnt die -- I believe this is at the beginning of Episode 2 Season 1, definitely Episode 2 though not sure on time stamp
Plus the 1st accountant working with Del, was working with the feds, so easy to assume Del new accountants would be washing cartel money since the other accountant went missing (murdered) and then Marty pops up in his place
and his main partner (cant remember his name, Bryce I think) I think they mentioned later on, when Marty got picked up by Petty that he was also working with the feds, the same one with 18 bugs in his apartment