Captain Oliver Queenan was a state official Captain in the Boston Police Department. He is first seen greeting Sullivan into the offices along his right-hand-man Dignam, even though he won't be working with them directly. Queenan and Dignam planned a breakout mission on Costello's mob to bring an undercover cop into the gang, using the recently gratuaded youngster Billy Costigan Jr.. He agrees, so Queenan's plan starts.
The meeting at 344 WashEdit
After steady work on the case of Costello, him and his gang finds out that there is indeed a "rat" in the mob. Feeling more and more petrified, Costigan plans a meeting with Queenan at 344 Wash down K street. After arriving, Costigan finds out that Queenan was tailed, by not only Sullivan's men, but Costello's too. So Queenan tells Costigan to leave, saying he can't protect him with him here. Being surrounded by mobsters, the Captain gets caught by the gang members, led up to the rooftop and thrown off.It's unknown if Queenan was dead before being thrown off the roof, since he made no sound for help, but it is stated that the fall is what that killed him.