5

When does Sullivan find out who the undercover guy (DiCaprio) was in The Departed? Is it when he says on the phone:

So it is you. Thank God you're all right. We were very worried.

And how exactly does he know it's Costigan?

Sorry, I've watched it a while ago and I'm trying to remember the story.

4

Sullivan is going through Queenan's belongings (he's been shot at this point) in his office and finds his cell phone. He uses the cell phone to call the informant. He still doesn't know Costigan's name or identity at this point, but he knows it's him especially when Costigan says

Let me talk to Dignam to confirm it.

after Sullivan tells Costigan that he's taking over the case.

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