Why don't the cops pursue Costello's crew after the shootout?

Question

In The Departed (2006), there is a brief firefight between Costello's crew and the plainclothes officers tailing Capt. Queenan after Queenan is thrown from the roof of 344 Wash.

All of the crew has prior offenses as is evident in Capt. Ellerby's presentation which features mugshots of all of them.

"Fitzy. Off-the-boat-psycho, lives with his mother, who's straight out of going my way. Delahunt, muscle. French, the number one."

Why don't the cops regroup and go after these guys later? They're known offenders, their pictures are in the system. Did the plainclothes officers not get a good look at them?

Answer 1

I see two main reasons for not rushing into Costello's hideout and chase these guys, after the death of Queenan and an officer gets shot.

1. They probably don't know where Costello's hideout is. Costigan is the only police officer who knows where the Costello crew hides and after Queenan's death, the only person who knows that Costigan is an officer is Dignam who definitely won't reveal the undercover agent's identity.

2. They are building a case, they want Costello, not his henchmen. And they also want to find who is the Costello spy working in the Special Investigation Unit. This piece of dialogue explains it:

   QUEENAN : Keep your act together. It's just a little while longer.
   BILLY: When are you going to take Costello? What's wrong with taking'em on any one of the million felonies you've seen him do. Get him for pissing in the street. Are you waiting for him to chop me up and feed me to the poor?
   QUEENAN: We're building a case. It takes time. You know that.
   BILLY: There's something wrong.
   QUEENAN: Maybe. Maybe.
   DIGNAM: Keep your ears opened. No bullshit.
   QUEENAN: I want you to listen for any chatter about a spy in the Special Investigation Unit. You hear anything like that?
   BILLY: Are you serious?
   QUEENAN: Hang tight for me. Just a little longer. We're this close.