Do You Like Hitchcock?

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In 1991 Giulio, a young boy bicycling in the woods, spots a woman darting between the trees. He follows her as she is joined by another woman and enters an old stone house. As he peers through the window, he witnesses the women wildly slaughtering a rooster. They spot the boy and chase after him, yelling "You'll be sorry! I'll kill you! You'll never get away from us!"
In present day, Giulio, now a young film student, is studying late at night when he spies on Sasha, an attractive neighbor girl as she undresses. The next morning, he makes love to his girlfriend Arianna. After she leaves, Giulio spies on Sasha as she argues with her mother. Later, while renting Fritz Lang films for his thesis, he sees her by chance at the video store. She tries to rent Alfred Hitchcock's Strangers on a Train at the same time as another woman, Federica. The two talk and strike up a friendship.
That night Giulio sees Sasha watching her movies in the apartment across the street. Her mother tells her angrily to turn down the volume. Sasha meanders out to the balcony where she briefly sees Giulio, who ducks back into his apartment.
The next day at the video store, Giulio sees Federica as she rents Strangers on a Train. The clerk, Andrea, makes advances towards her, but she's not interested and drives away. Later, while riding his bicycle, Giulio sees Sasha and Federica having lunch by a fountain, talking closely.
In the next scene, an intruder uses a key to enter Sasha's apartment while her mother is home alone watching TV. The intruder murders Sasha's mother with a brass pestle. Giulio is awakened later that night by the sounds of the police investigation as they question Sasha.
Giulio meets Andrea the next morning to talk briefly about the murder, after which they both attend Sasha's mother's funeral.
Giulio meets Arianna for lunch. Reading the newspaper, he finds out that Sasha Zerboni was an only child and her mother was a rich widow. Giulio tells his girlfriend that he's been thinking about Strangers on a Train all day. They watch it together and he tells Arianna that he suspects that Sasha and Federica are in an agreement similar to the one in the film. During the course of their conversation, Arianna realizes that he's been watching Sasha out his window.
Later, Giulio sees a woman cleaning Sasha's mother's apartment. He goes across the street and finds the cleaning crew mopping up the bloody crime scene. After a brief exchange with the crew chief, Giulio dashes out, snatching a letter out of the mailbox. The letter is a bank statement showing almost 1 million Euros in the Zerboni's account. Returning to his apartment, he is surprised to find his mother cleaning it. She tells Giulio that she wants him to meet her new fiancÃ©, her tennis coach.
Upon Sasha's return, the cleaning crew chief tells her about the visit of a young man from across the street. Sasha knows it's Giulio.
While Giulio showers, someone breaks into his apartment. The intruder breaks an empty wine bottle and flees when Giulio hears the noise and jumps out of the shower. Giulio calls Arianna and asks her to come over to spend the night because he's nervous. Ariana wants to make love but Giulio wants to discuss the situation with her but she's more interested in making love. When he insists that it is now Sasha's turn to kill someone close to Federica (Ã  la Strangers on a Train), she says he needs to see a shrink and storms out.
The next day, Giulio has his locks changed. As he passes by the video store, Andrea tells him that Sasha had been in and asked about him, saying that she knew he watched her.
That night, Giulio has a nightmare about his boyhood experience with the two women slaughtering the rooster. He awakes to hear someone trying to break into his apartment, but the intruder runs away before Giulio can get the door unlocked and give chase.
The following day, Giulio returns Strangers on a Train to the video store. Andrea asks Giulio to mind the store while he steps out. Giulio agrees and while Andrea is away, he looks up Federica's address on the store computer. In the middle of his poking around, Sasha comes into the store and they speak briefly. She sees that he has returned Strangers on a Train and she puts it back on the shelf.
Giulio rides his motorbike to Federica's apartment and follows her to her workplace. Through the office window, he sees Federica's boss grope her inappropriately. At the end of the day, Federica hesitantly leaves with her boss, clearly unhappy. Giulio follows them to the boss's apartment and sees them arguing in a second floor window. Giulio scampers up a ledge to a second floor balcony as it starts to rain. He can see and hear Federica and her boss arguing in bed. The boss is blackmailing Federica for sexual favors over money she stole from a client.
As the boss's forces himself upon Federica, he spots Giulio looking through the window. Giulio tries to scamper back down the ledge, but falls and fractures his ankle. The boss gives chase and Giulio limps to his motorbike, barely escaping.
Giulio's mother takes him to the doctor who tells him he'll have to be in a cast for several weeks. Arianna comes over to his place to help him mend, but when he starts telling her what happened and his theory of how Federica wants Sasha to kill her boss, Arianna says he's crazier than ever and storms out, saying she never wants to see him again.
Andrea comes to visit with some Hitchcock movies for Giulio to watch while he's laid up. He uses the bathroom and a few minutes later tells Giulio that the tap is stuck and won't turn off. When Giulio enters the bathroom, Andrea grabs him and tries to drown him in the tub. While Giulio and Andrea are struggling, Giulio's mother and her fiancÃ© come to visit Giulio. Hearing the screams from inside the apartment, the fiancÃ© breaks down the door and pulls Andrea off Giulio. While Giulio's mother revives Giulio, the fiancÃ© chases after Andrea who flees into the street and is fatally hit by a car. Sasha looks down from her balcony, realizes what happened and becomes physically ill.
The police arrest Sasha and detectives speak with Giulio. Later, Arianna comes over and they make up. Giulio explains that Sasha's plot was more similar to the film Dial M for Murder in which a man hires a killer to kill his wife. Here, Sasha hired Andrea to kill her mother.
As Arianna is closing the window, she sees a woman dressed in black approaching Sasha's apartment. Despite Giulio's protests, Arianna insists on chasing after her to see what she's doing. Giulio sees through his binoculars that the woman is Federica in disguise. She is looking for the key which she mistakenly left in Sasha's apartment. Giulio realizes that Federica was the real killer of Sasha's mother and that Andrea must have joined them later. Giulio calls Arianna on her cell phone, but she refuses to give up the pursuit despite his protests that Federica is a killer.
Giulio calls the inspector who responds immediately, arriving at Sasha's apartment before Arianna has a chance to confront Federica. They chase her up to the rooftop where she threatens to jump off. Federica slips off the edge of the building, but the inspector catches her by the arm before she falls. The inspector and Arianna manage to haul Federica up to safety, at which point she is placed under arrest.
In the final scene, Giulio's cast is off and he is exercising. Giulio gets a phone call from his mother and while they talk he sees a light go on in Sasha's former apartment. Another beautiful woman has taken residence in the apartment. She is almost naked and reading a book on the couch. She sees Giulio watching her from across the street, but doesn't seem to mind. Giulio stands staring with mouth agape.

Question: Who killed Sasha's mother?
Answer: Frederica