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, 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. This causes panic and dissent among the city, and results in innocent patrolmen either being shot to death by terrified civilians or avoided on the streets by people afraid of them being the Maniac Cop.
Ellen Forrest, suspecting that her husband Jack may be the Maniac Cop, follows him to a motel and catches him in bed with fellow officer Theresa Mallory. Distraught, Ellen runs out of the room, and is slain by the killer. 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; when they shoot him several times, the killer appears unfazed.
Mallory hides out in McCrae's apartment while he investigates Sally Noland, 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, an officer who was unjustly put into a maximum-security prison at Sing Sing Correctional Facility in the village of Ossining, New York after being framed for police brutality when he was closing in on corruption in city hall. He was mutilated and killed in a shower room in Sing Sing by other inmates, whom he helped incarcerate.
When McCrae and Mallory visit Jack, they tell him 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 convinced that Cordell is going to turn on her. After finding an officer hanging from the ceiling, Sally is beaten to death by Cordell. Hearing the commotion, Jack and Mallory leave the interrogation room and find the corpses of numerous officers strewn about the halls of the building. Mallory goes to McCrae's car while Jack 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 and Captain Ripley about Cordell, but the two refuse to believe her and have her arrested. Cordell appears and fatally stabs Pike and Ripley, 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. Cordell attacks Mallory and Jack, kills the other officer, and tries to escape in the van when backup arrives. Jack clings to the side of the van and fights for control of it, causing Cordell to drive into a suspended pipe, which impales him. Cordell loses control of the vehicle, which crashes into the river, and sinks. Afterwards, the van is fished out; as it is searched, Cordell's hand emerges from the water.
Later in the extended version, the mayor, confident that Cordell is dead, relaxes in his office. After the mayor's assistant leaves the office, Cordell silently appears from behind the curtain and kills the mayor as revenge for framing him.