Quite possibly texts, e-mails, phone logs, banking records... If Smollett did set the whole thing up, these brothers probably gave CPD everything in return for no jail time.
It's also possible the guys refused to beat Jussie and backed out of the 'deal'. That would explain why Smollett had no injuries beyond a little scratch.
Our the cops let them go and will record their calls and catch them talking about this with fagman.