|Plot details follow, read at your own risk.|
Events of L.A. Noire
Petersen initially approached Jack Kelso with a job offer as DA's investigator, while Jack was recuperating in the hospital after being assaulted by Leland Monroe's men. While Petersen wanted Kelso to target the Vice squad, Kelso instead offered him the bigger target, the Suburban Redevelopment Fund, which was comprised of many of the city's corrupt leaders. Kelso explained to Petersen the conspiracy behind the SRF and Monroe. Petersen supported Kelso's investigation as it gave him a chance to bring down the whole administration.
Petersen was present as both Phelps and Kelso entered the river tunnels in search of Elsa Lichtmann and Ira Hogeboom. As the two heroes rushed in, Petersen held off corrupt Police Commissioner Worrell. Petersen warned Worrell that the truth of the conspiracy would be exposed and offered a chance to bargain his way out to avoid implication.
Petersen later attended the funeral of Cole Phelps along with Worrell, Mayor Bowron and Vice Detective Roy Earle. Although the SRF, Monroe and Benson were undone, it would seem that Petersen's deal involved concealing the corruption of most of its members in exchange for the position of District Attorney, replacing Donald Sandler. Ironically and unfortunately, Petersen had abandoned his crusade against corruption and collaborated with the very source he desperately wanted to stop. He is last seen shaking hands with Earle after the latter delivered Phelps' eulogy.