The Prowler

During the second world war a woman named Rosemary Chatham writes a Dear John letter to her boyfriend, breaking up with him due to being away too long. On June 28, 1945, Rosemary (Joy Glaccum) is attending a graduation dance in the town of Avalon Bay with her new boyfriend Roy (Timothy Wahrer), who suggests they go out to the point in order to make out. While there, they are attacked by a mysterious prowler in an army combat uniform, who impales them both with a pitchfork, leaving behind a rose.
Twenty years later; on June 28, 1980, Pam MacDonald is organizing the first graduation ball in 35 years with her friends Lisa, Sherry and Sherry's love interest Carl. That afternoon, while visiting her love interest Mark London, who is the town's deputy, she overhears a report of a prowler, who may be on the way to Avalon Bay. The Sheriff, off to a cabin retreat leaves Mark in charge of keeping order in the town and at the dance while he is away. That night, after Pam leaves for the party, Sherry receives a surprise visit from Carl, who goes to undress to join her in the shower, but he is attacked and has a bayonet shoved through his skull. The killer then moves into the bathroom and impales Sherry with a pitchfork. At the dance, Pam becomes jealous when Mark is pulled into a dance with Lisa and leaves when he accidentally spills his drink onto her dress. She returns to the dorm to change but is chased by an unseen prowler, eventually running into Major Chatham; an old man bound to a wheelchair, reuniting with Mark, he investigates but does not discover the bodies of Sherry and Carl. They go together to investigate the major's home, not finding him there, Pam realizes that his daughter Rosemary's killer had never been found. Convinced the prowler from earlier is the one responsible, Mark heads with Pam to the dance to warn the chaperone Miss Allison about the possible danger. Meanwhile Lisa, fed up with her boyfriend Paul for getting drunk and sick, goes out to a nearby pool to cool off. Paul is arrested by Mark for public intoxication, meanwhile Lisa encounters the killer while swimming, who mercilessly slices her throat open. Allison learns that Lisa had gone out and goes to find her, but she is stabbed through the throat as she tries to escape the killer. Mark tries to call the cabin that the sheriff went to, but is ignored by the site worker too lazy to check to see if the sheriff is awake.
The local shopkeeper Otto complains to Mark he witnessed a disturbance in the cemetery, Mark and Pam go to investigate and discover an opened grave with Lisa's body in it. Mark tells her that the reported prowler had been caught hours before Lisa's murder and Pam suspects this is the same killer who murdered Rose and her boyfriend in 1945. They go to investigate Major Chatham's house once more, Mark is attacked and left for dead as the Prowler then chases Pam through the house. Otto appears and shoots the attacker, who recovers and shoots him dead before attacking Pam. During the scuffle, Pam discovers the Prowler is none other than the sheriff himself and turns his gun against him, blowing his head clean off. The next day, Mark returns with Pam to her dorm and she goes up alone. Discovering Sherry and Carl's bodies in the shower, she screams as he seems to come to life and grab at her, only realizing that he is dead, and that him grabbing at her was a hallucination.

Question: Who killed the Prowler?
Answer: Pam