I think it would depend on the type of illegal activity involved. Most leases have language prohibiting illegal activity. You'd have to review the contracts involved. I think the only responsibility of an agent/broker would be to inform their clients first then possibly law enforcement if appropriate. You would have to prove that there was knowledge & approval of the activities. That's not an easy thing to prove 100%. You'd probably have to consult a local attorney to explore possible liability.
Most property owners I know wouldn't condone or allow illegal activity due to the liability involved. Property owners have lost property to the state in cases of blatent drug dealing etc.