Maniac Cop

In New York City, a waitress on her way home is assaulted by two muggers and seeks aid from a police officer, who breaks her neck. Over the next two nights, this "Maniac Cop" commits more murders, prompting Lieutenant McCrae (Tom Atkins), who was told by his superiors to suppress eyewitness accounts that the killer was wearing a police uniform, to pass on information to a journalist, in an attempt to protect civilians. Unfortunately, this causes panic and dissent among the city and results in innocent patrolmen being shot to death by paranoid people.
Ellen Forrest (Victoria Catlin), who suspects that her husband Jack (Bruce Campbell) may be the Maniac Cop, follows him to a motel, where she catches him in bed with a fellow officer, Theresa Mallory (Laurene Landon). Distraught, Ellen runs out of the room, and is slain by the murderer. Jack is arrested under suspicion of murder, but McCrae believes Jack has been framed. McCrae gets Jack to tell him about his relationship with Mallory, who is attacked by the Maniac Cop while working undercover as a prostitute. Mallory and McCrae fight off the killer, who is deathly cold even through his gloves and does not appear to breathe; though they shoot him several times, the killer appears unfazed.
Mallory hides out in McCrae's apartment while he investigates Sally Noland (Sheree North), the only person Mallory told about her affair. McCrae follows Noland to a warehouse, where she meets with the maniac cop and refers to him as "Matt". Returning to police headquarters, McCrae discovers files on Matthew Cordell, a fellow officer who was imprisoned in Sing Sing for police brutality and closing in on corruption in city hall. While McCrae is looking into his past, Cordell flashes back to being mutilated and killed in a shower room in Sing Sing.
When McCrae and Mallory visit Jack, they tell him that they think Cordell is the real killer and plan to visit the chief medical examiner at Sing Sing. McCrae leaves to go to the clerical room, and he is attacked by Sally, who is in hysterics, convinced that Cordell is going to turn on her. After finding a policeman hanging in a noose, Sally is grabbed by Cordell and beaten to death. Hearing the commotion, Jack and Mallory break out of the interrogation room and find the corpses of numerous officers strewn about the halls of the building. Jack tells Mallory to go to McCrae's car while he searches for Cordell, who disappears after throwing McCrae out a window, killing him. Jack, who looks like the one responsible for the carnage to responding officers, flees with Mallory.
The two go to see Sing Sing's medical examiner, who admits that while he was preparing to autopsy Cordell, the officer showed faint signs of life. The examiner secretly released Cordell into Sally's care, convinced he was completely brain dead. During the 50th Annual St. Patrick's Day Parade, Jack waits outside as Mallory warns Commissioner Pike (Richard Roundtree) and Captain Ripley (William Smith) about Cordell, but the two refuse to believe her and have her arrested. Cordell stabs Pike and Ripley to death, then targets Mallory, knifing the policeman left to guard her. Mallory escapes through a window, while Jack is arrested and placed in a van, which Cordell hijacks.
Mallory and another officer chase the van, which Cordell takes to his warehouse hideout, running over the watchman on the way in. Cordell attacks Mallory and Jack, kills the other officer, and tries to escape in the van when backup arrives. Jacks clings to the side of the van and fights for control of it, distracting Cordell and causing him to drive into a suspended pipe, which impales him. Cordell loses control of the vehicle, which crashes into the river, and sinks. The van is fished out, and, as it is searched, Cordell's hand shoots out of the water. Everyone then realizes that Jack Forrest didn't comitt the murders.
In the extended cut, corrupt mayor Jerry Killium relaxes in his office, content Cordell is gone. After Killium's assistant leaves, Cordell, who was hiding behind a curtain, murders the mayor offscreen as the credits roll.

Question: Who was hiding behind a curtain?
Answer: Cordell