I don't think this was the CD with the recordings. That one came in an envelope mailed through the post office from Costigan to Sullivan.
The envelope he gave to Madolyn was blank and she wrote his name on it with a ballpoint pen. I think it's implied that what was in there was a written message explaining who the rat is to Dignam, whenever he turned up again (remember, Costigan was having trouble finding Dignam). It was to be given to Dignam as a last resort should Costigan not be able to bring Sullivan to justice himself.
Wasn't it Costigan's identity?? Remember that scene in the office when Matt Damon erase his police record? I think it was a copy of his record.
shareCostigan doesn't know his identity was erased until the end when he handcuffs matt damon, at this moment matt damon threaten him and tell him he erased his identity and he will never prove anything in front of a jury.
Sadly Costigan dies a few seconds after.
Yeah, this is what he said, but is it really the case?
shareDignam was the only person on the planet who knew the full extent of Billy's activities.
What I want to know is how Dignam found all of this out or how Madelyn found him?
Costigan probably had details in the envelope that he gave to Madeline about how to reach Dignan, and get the envelope (or at least the info it contained) to him.
shareI took it as the envelope billy gave her must have had the details where those boxes of tapes are and probably a request to give that info to dignam. Because he can't trust anyone from unit to hand it over to dignam, he does not know how many or who's corrupt. He's says that she's the only one he can trust while giving the envelope. Madolyn was not connected to them and she could have located dignam under official capacity being a police shrink.
shareA LETTER FROM DOC BROWN...THE POST OFFICE HAD BEEN HOLDING IT FOR OVER A HUNDRED YEARS.
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