In The Departed, a rat is embedded within the Massachusetts State Police (MSP) by the Irish Mob, but the movie never reveals how Staff Sergeant Sean Dignam (Mark Wahlberg) learns that Sullivan (Matt Damon) is the double-crosser. The Departed follows Billy Costigan Jr. (Leonardo DiCaprio), a recruit tasked with infiltrating the mob to determine the identity of their man inside the MSP. That rat is Colin Sullivan, who was groomed to rise through the ranks and become a top cop so that the mob will always be one step ahead. Billy successfully learns Sullivan is the rat, but he’s murdered by another informant Frank had in the MSP.
The Departed famously ends with almost everybody dying, and in the very final minutes, when it seems like Sullivan has gotten away with his crimes, he enters his apartment to find Dignam, who shoots him dead. Dignam is a member of the MSP’s Special Investigations Unit who is suspended when he loses his cool following the death of his supervisor, Captain Queenan (Martin Sheen). It’s assumed that Dignam knows Sullivan is the rat and is getting revenge for Queenan’s murder. However, while that makes for a shocking twist, it doesn’t explain how Dignam learned Sullivan was the rat, as Costigan was the only one who knew.
Madolyn Possibly Went To Dignam With The Information
Madolyn (Vera Farmiga) is a police-appointed psychiatrist and Sullivan’s romantic partner, yet she’s completely unaware of his connection to Irish Mob boss Frank Costello (Jack Nicholson). She also becomes Costigan’s therapist and has an affair with him. Before Costigan sadly dies in The Departed, he gives Madolyn a recording of Sullivan and Costello. He also gives her an envelope that is never fully explained but which could have encouraged her to go to Dignam. Given that Sullivan lies to her, and he’s responsible for Costigan’s death, and she has a strong moral compass, she was never going to ignore Sullivan’s crimes and stand idly by.
Dignam Might Have Been Working With Costello
Another theory is that Dignam knew Sullivan was a rat because he also was an informant for the mob. Yet another possibility is that he was the point man for a certain rat within the mob. It’s revealed in The Departed that Costello covered his back by being an FBI informant. However, the movie doesn’t reveal how he reports to the Bureau. According to a popular Reddit theory, Costello went through Dignam. It’s something that Mark Wahlberg’s proposed but unmade The Departed 2 could’ve detailed, but the idea is that Dignam may have been working more closely with Special Agent Frank Lazio (Robert Wahlberg — Mark’s brother) than it seemed.
Costello was protected, after all, and someone in the Massachusetts State Police might have needed to know that. In one scene, Queenan, Dignam, and Costello chat openly, and while Dignam doesn’t have the same status on the force as Queenan, he and Costello talk as if they’ve known each other for a while. Either way, as Sullivan was the only other person in the Costello case who survived and knew the mob boss was working with the feds, it makes sense for someone with such dangerous information to be quietly killed. It’s likely Dignam’s stealth killing at the end of The Departed came from orders from above.
