Question: When Costigan finds out that Costello is an FBI informant, why doesn't Costigan speak privately with Costello and reveal to him (Costello) about being an undercover cop in order to get Costello to tell him (Costigan) who the moles are in the police department? Then he (Costigan) can inform Dignam and Queenan, so they can have the moles (Sullivan (and Barrigan)) arrested. If Costello is an FBI informant and Costigan is an undercover cop then they are on the same side, so theoretically they can trust each other.
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Answer: Just because Costello is an FBI informant does not mean he is a nice guy. Costello would probably kill Costigan if he knew he was a cop. They are not on the same side at all. Costello also benefits greatly from having Sullivan as a police officer that is loyal to him, so he would never roll over on him.
oldbaldyone
They aren't FBI informants either. They are Boston State Trooper undercover agents.