He planted a bug on Peck, that caused interference with the bug Reese planted and allowed Reese to discover that Fox was surveilling Peck as well. Reese followed him, but Fox was able to escape in a planted truck.
He then ambushed Peck in his apartment and attempted to kill him, however, when Reese interfered, he was forced to flee.
Since none of the attempts to kill Peck had worked due to Reese following and protecting Peck, Fox and his team members were given the order to kill both Peck and Reese in another attack.
The team then ambushed the building where Peck worked and tried to shoot him but Reese managed to protect Peck again.
Eventually, Fox was able to track down the car Peck and Reese were in and attacked it with a sniper, causing the car to wreck. Unaware that Peck could escape he tried to kill Reese but in a final fight Reese attacked him and killed him by stabbing a knife in his throat. Before he died Fox confessed to Reese that he knew nothing about why he was ordered to kill Peck. (“No Good Deed”)