In 4.10 "The Thing About Heroes...", Mac continues his Chicago investigation and follows the clues to a body hanging in an unused floor of the Chicago Tribune building. A hangman puzzle written on the wall leaves out the letters that spell "Coward". The decomposed body is revealed to be that of Bobby Toole who died thirty years ago. Mac goes and talks to a former friend of his named Jimmy. He asks Jimmy if he's the stalker and if it's because of his brother Will's death. Mac has proof the bloody T-shirt is Will's and says they were the only ones who knew that Bobby Toole killed Will and that they killed Bobby. Jimmy is incensed at the idea and that Mac is acting high and mighty after he apparently let Will down that night. Jimmy says Mac has no idea what it was like seeing his father cry and lying to his younger brother Andy about Will's death. Then Jimmy storms away. Mac turns around to see Flack standing there. Since an attempt was made on the team's life while Mac was away, and the chief had been getting angry calls about Mac's work from Chicago police, Don has been sent to help Mac clean things up quickly. Mac tells Flack when he and Jimmy were both 14 they used to tag along with Jimmy's brother Will (age 16) to make deliveries for a guy named Sal Marchetti. One night, they delivered money to Bobby Toole, but Toole got upset that it wasn't all there. He began to beat Will. Jimmy grabbed Bobby's gun but was hit and dropped it. Jimmy yelled for Mac to get the gun and he did but Mac couldn't bring himself to pull the trigger. Jimmy grabbed the gun from Mac and shot and killed Toole. They took Will to the ER where he died and told the police they were mugged, but they didn't see by who. They then told Sal what happened and Sal told them he would help hide the evidence.