Billy was desperate. He had planned to contact the cop that he had known quite well at Cop School, Detective Brown, and ask him to contact Dignam so that Brown could help Billy arrest Sullivan. But then a bad nights dreaming had made Billy realise that even Brown could be corrupt as Sullivan must pull a lot of weight, and even if not corrupt, could Billy be sure that Brown would contact Dignam. Billy's really didn't think that Brown could be corrupt, but he was the weak link in Billy's plan, so..
Delaying increased the risk that Sullivan might target Billy, but Billy had to be sure about Brown. A night spent at a dive frequented by some SIU cops confirmed Billy's worst fears, Bown worked directly for Sullivan. Brown could not be trusted. There was no way that Billy could contact the SIU with Sullivan not knowing and being able to finger Billy, either directly or via any stooges Sullivan had.
Unless Sullivan was in no position to finger Billy...
The next evening Billy went round to see Maddy before she left work to go to the cop she was to marry. Billy was gutted by that thought. Billy gave Maddy a letter which was his insurance policy. The letter contained a note to Maddy explaining what to do : if Billy was in trouble go to Dignam and ask him for help. The letter to Dignum listed everything that had happened since Queenan was murdered along with a warning not to trust the FBI. Maddy was to ensure that Dignum, who Billy now felt had run out on him, would help Billy. If Dignum would not help Billy then Dignum would not be told of Billy's evidence stash, and there was a letter to the New York Cops. Hopefully they would help Billy, or avenge him on Sullivan and Dignum. Maddy took the letter, she seemed troubled. Billy warned her not to say anything about the letter, even to her fiancé, since it involved police corruption at the highest level; if the corrupt cops knew about the letter they would go after her, so the less she knew the better. Billy said that he was going to take his evidence to the SIU, but at great risk.
The next morning Maddy, who started work later than Sullivan, checked the mail and was intrigued to find a package from Billy. Intrigued, and with a sense of foreboding, she took out the CD and played it. What she heard shocked her to the core. After a minute Maddy took the package, repackaged the CD, resealed the package, gave it to Sullivan, and then went to pack her things. Sullivan soon had more to occupy his mind than Maddy's silent behaviour. An hour later Sullivan phoned Billy. Billy told the rat faced faggot to meet him at the rear of one of the central Boston shopping mall's Men's Public Conveniences, the rear being quiet and somewhat suited for dubious business.