Maniac

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Frank Zito (Joe Spinell) awakens in his bed and screams and cries after having a nightmare about killing a couple on a beach. Once awake, Frank dresses and leaves his resided one-room apartment (consisted of paintings, one being a framed picture of his deceased mother, and a collection of mannequins) towards downtown Manhattan. Frank passes by a hotel with two prostitutes standing outside; one of them invites Frank and checks in. Inside, the prostitute (Rita Montone) and Frank kiss, where abruptly Frank strangles her and later carefully scalps the woman with a utility razor, while on a range of disturbance. Frank returns home and adds the killed prostitute to his mannequin collection by placing her clothing and nailing the scalp onto the mannequin. Inside his mind, he tells himself that beauty is a crime punishable by death. After Frank finishes, he reads a newspaper detailing about the murdered couple at the beach as seen in Frank's nightmare. Sometime later, Frank dresses and takes a collection of weapons with him, including a double-barrelled shotgun, before leaving.
Driving around Brooklyn and the Queens area, Frank finds a couple exiting a local disco and parked near the side of the Verrazano Bridge. The disco boyfriend (Tom Savini) starts up the vehicle after his date uncomfortably sees a spying Frank. Frank appears in front of the vehicle and kills the couple with his shotgun. Frank later returns and adds the killed woman to his mannequin collection as he watches the recent murder on television. Soon, Frank begins talking to himself and the other mannequins as he begins to cry himself to sleep.
The next day in Central Park, Frank follows a woman named Anna (Caroline Munro) after she takes a photo of him and a little girl riding on a bicycle in the distance. At night, Frank finds two nurses leaving the Roosevelt Hospital, with one of them picked up by her boyfriend and leaving the other nurse (Kelly Piper) alone. Frank then stalks the nurse, but she notices him and escapes into a nearby subway station. She tries to board a leaving train, but performs unsuccessfully and hides into a bathroom stall. Unbeknownst that Frank still awaits, he kills her with a bayonet. Frank then returns with another mannequin to the collection and talks to himself as he decorates the mannequin.
Days later, Frank later heads to Anna's apartment, where she is developing the photographs including the photo of Frank. After telling her about him being in the photo, she invites Frank inside and discuss Anna's work as a photographer. He notes that all her models are women, and she hopes to sell some of her photographs of them, where Frank suggests to preserve them. Frank asks Anna out for dinner and she accepts. During dinner, Frank shows Anna his deceased mother's photo and explains to her she was killed in a car accident years ago. Frank asks Anna she wanted to go out again sometime and she invites him to an art gallery on Thursday. A few days later, Frank arrives at a studio where Anna is taking photos of models at a photography session. On break, Frank gives Anna a stuffed bear as a gift and she introduces him to a model named Rita (Abigail Clayton). Anna and Rita continue shooting, and Frank sees them talking and holding hands together; he then steals Rita's gold necklace left aside and leaves. In the evening, Rita returns to her apartment and takes a bath, where Frank arrives and gives Rita her necklace. Unbeknownst, Frank gains access inside and attacks, where she is tied to her bed. Frank begins talking, addressing her as his mother, and stabs her with a switchblade. Frank scalps her, adding to his collection, and eventually attends Rita's funeral with Anna.
Frank calls Anna and invites her to the theatres and picks her up. Frank then drives Anna to a cemetery, wanting to put flowers on his mother's grave. At the cemetery, Frank lays the flowers beside his mother's headstone. Frank mourns over one of his early victims, and attacks Anna by her neck. As Frank searches, Anna wounds him with a shovel and flees. Frank returns to his mother's grave, where he hallucinates about his decomposing mother attacking him. A saddened Frank returns to his apartment, where suddenly his mannequins come alive and mutilate Frank with his weapons, ultimately tearing off his own head.
The next morning, two police officers appear (having been called on about the recent murders) and burst into Frank's apartment upon Frank on his bed, who committed suicide from the hallucinated event. As the officers leave the scene, Frank's eyes suddenly open as the film ends with the title printed onto the screen.

Question: What attacks Frank in his apartment?
Answer: Mannequins