Plot hole

Rating: Null/10 (7 April 2007)

Warning: Spoiler.
Overall an excellent movie, and certainly Scorsese's best for a long time. However there is one obvious plot-hole which really spoiled the last third of the film for me.
It's this: when Capt Queenan dies, why doesn't Costigan simply call Dignam and tell him that whoever ordered the tail on Queenan must be the informant? Costigan actually explains this to Queenan immediately before his fall from the roof, so he is clearly aware of the significance of the tail, yet as soon as Queenan dies, Costigan fails to take this most obvious course of action.
Then Dignam's two weeks leave at this crucial junction feels contrived- a device to remove any protection from Costigan for the remainder of the film. The logic of the plot however should lead to the informant being found almost immediately- either via Costigan, Dignam (who cannot conceivably fail to make contact with Costigan even if Costigan doesn't call him), or by Alec Baldwin's character, who would surely order that Costigan be pulled out asap to sort out what just happened.