They won’t ‘come clean’ until it gets to the point of negotiation:
Either your or Mr./Mrs. X is going to prison. First one who talks gets the one and only deal.
Don’t these guys watch TV detective/lawyer show?
Where’s Deep Throat when you need him (her)? People with information are so scared they dare not speak. They don’t have to be threatened. They know that silence is the currency of the realm.