Friday the 13th Part VII: The New Blood

As a young child, Tina Shepard witnesses her alcoholic father physically abusing her mother. The emotional trauma unlocks Tina's previously latent telekinetic powers, which result in her father's death at the bottom of Crystal Lake. Ten years later, Tina, now a teenager, is still struggling with the guilt surrounding the death of her father. Tina's mother, Amanda, takes her to the same lakeside residence so that her powers can be studied (and unknowingly exploited) by her psychiatrist, Dr. Crews. Dr. Crews begins a series of experiments (verbal assaults) designed to agitate Tina's mental state, forcing her powers to become more pronounced.
After a particularly upsetting session with Dr. Crews, Tina runs from the cabin and to the dock thinking about her father's death. While thinking about him, she wishes he would come back. Instead, Tina's psychic powers breaks the chain around the neck of Jason Voorhees, the undead Crystal Lake serial killer. Tina's torment from her powers is increased as Jason emerges from the lake and his reign of terror descends on the area once again.
Next door to the Shepard residence is a group of teens who are throwing a birthday party for their friend Michael: preppy Russell and his girlfriend Sandra, Ben and his girlfriend Kate, science fiction writer Eddie, stoner David, perky Robin, shy Maddie, and snobby socialite Melissa. Michael's cousin Nick, who has arrived just for the party takes a liking to Tina, much to the jealousy of Melissa, who bullies Tina. Tina tells Nick about Jason and has a vision of Jason murdering Michael. Meanwhile, Jason kills Michael in real life, along with Jane, and later kills another couple camping in the woods.
While Tina goes off to find her mother and Nick goes with her, Jason proceeds to kill the other teens. Russell is killed with a hatchet to the face, and Sandra is pulled underwater while she is skinny-dipping. Jason stalks Maddie and kills her in the barn with a scythe, and then crushes Ben's skull with his bare hands before shoving a party horn into Kate's eye. David, Robin, and Eddie are also killed. When Jason attacks Dr. Crews, he saves himself by selfishly using Mrs. Shepard as a shield while Jason kills her instead. Crews and Tina meet up again and Crews tries to lie and tell Tina her mother is fine before Jason finds and kills him. Tina finds her mother's body shortly afterwards and uses her powers to electrocute Jason and to crash the house down on him. She tells Nick and Melissa what happened, but Melissa thinks she is crazy and goes to leave the house, only for Jason to return and kill her with an axe to her head.
Nick tries to fight off Jason but is subdued. Tina unleashes her powers in various ways such as forcing Jason's mask to tighten until it breaks in two, revealing his decaying and deformed face. As the battle rages on, the Shepard lakeside cabin is destroyed by an explosive fire and trio are thrown back into the dock. Although she is unable to kill Jason, she unknowingly summons the spirit of her father, who emerges from the lake and drags Jason back down into the depths with him, chaining the killer once more.
The next morning, Tina and Nick are taken away in the ambulance and the screen fades to black while Jason's whispers can be heard from far away.

Question: Who killed the other teens?
Answer: Jason