How did Matt Damon's partner know he went to meet Leonardo DiCaprio?

In the end of The Departed, Matt Damon's partner comes to his rescue, shooting Leonardo DiCaprio's character along with the friend whom Leo had asked to come to Damon's arrest as protection.

My question: How did the partner know to come? Damon looked stunned when the character showed up (he was in fact begging DiCaprio 'kill me') and the character further gives away to the audience that Damon didn't know he was also working for Costello ('you think you're the only guy he had on the inside?').

Best Answer

Billy (Di Caprio) called Trooper Brown and told him to meet him on the roof. He did this as Brown knew Billy from their time together in police training and that he could help verify that Billy was indeed an undercover agent.

Brown shows up and finds Billy trying to arrest Colin.

Its not made clear in the film as to how Barrigan (Franks other man on the inside) knew where to find Colin but as Brown and Barrigan worked together I would assume he was either with Brown when Colin called him or Brown told Barrigan as to where he was going. Its also possible that Brown brought him as backup.