A serial killer, Clay Dobson, whom Mac helped put away five years previously, comes back to haunt him after the man is released, thanks to Mac's arrest of the detective (Dean Truby) who took his confession. Mac's single-minded intensity in his renewed pursuit of this killer makes him short-tempered with his concerned coworkers. After the CSIs discover one of Dobson's victims is still alive, Mac charges after him, alone, cornering him on a roof of a tall building. Dobson comes crashing to earth moments later, fatally impacting on the hood of a police cruiser, only feet away from Flack and some uniformed officers arriving as backup. His hands are cuffed, and Mac gazes down from the roof in horror (episode 3.21 "Past Imperfect"). Flashbacks in the following episode reveal that Dobson intentionally fell from the roof, telling Mac that if he went down, he would take the detective with him. The Chief of Detectives, Brigham Sinclair, in a bid to remove Mac from his position at the lab (for political reasons, as Mac believes), initiates an Internal Affairs investigation, even though the district attorney did not find enough evidence to charge Mac (episode 3.22 "Cold Reveal"). During the hearing, the prosecutor appears to be determined to destroy Mac's career, despite attempts by his colleagues to aid him in their testimony. Former Detective Truby calls Taylor from jail, desiring to meet him in person. During the visit, Truby, guilty over Dobson's release, offers Mac a trump card to play against Deputy Inspector Gerrard and Sinclair: When Dobson had originally been arrested several years previously, Gerrard, then a lieutenant, failed to remove Dobson's belt. Dobson used the belt in a suicide attempt in his cell. Gerrard and Sinclair, the precinct captain, covered up the suicide attempt as well as Gerrard's lapse in procedure. Mac confronts the two and threatens to take his evidence to the media, thereby ruining both their political aspirations. Sinclair decides to have Mac cleared of all charges and the Internal Affairs investigation discontinued, and Mac remains quiet about his knowledge.