Inferno

Rose Elliot (Miracle), a poet living alone in New York City, finds an ancient book called The Three Mothers. It tells of three evil sisters who rule the world with sorrow, tears, and darkness. The book, written by an architect named Varelli, reveals that the three dwell inside separate homes that had been built for them by the architect in Rome, Freiburg, and New York. Rose suspects that she is living in one of the buildings and writes to her brother Mark (McCloskey) in Rome, urging him to visit her. Using clues provided in the book as a guide, Rose searches the cellar of her building and discovers a hole in the floor which leads to a water-filled ballroom. She accidentally drops her keys and enters the water to find them. After she reclaims the keys, a putrid corpse suddenly rises from the depths, frightening her.
In Rome, Mark attempts to read Rose's letter during class. He is distracted by the intense gaze of a beautiful student (Ania Pieroni), who leaves suddenly; Mark follows, leaving the letter behind. His friend Sara (Eleonora Giorgi) picks up the letter, and later reads it. Horrified by the letter's contents, she takes a taxi to a library and finds a copy of The Three Mothers. Sara is attacked by a monstrous figure who recognizes the book. She throws the book to the ground and escapes. She seeks the company of a neighbor, Carlo (Gabriele Lavia) and both are stabbed to death by a gloved killer. Mark discovers the bodies and two torn fragments from Rose's letter. After the police arrive, he walks out of Sara's apartment and sees a taxi slowly driving by. In the back seat of the vehicle is the music student, staring at him intently again.
Mark telephones Rose but is unable to hear her clearly. He promises to visit just before the connection fails. Rose sees two shadowy figures preparing to enter her apartment. She leaves through a back door, but is followed. She is grabbed from behind by a clawed assailant and brutally guillotined across the neck with glass from a broken window.
Arriving in New York, Mark meets some residents of Rose's building, including a nurse (Veronica Lazar) who cares for elderly Professor Arnold (Feodor Chaliapin, Jr.), a wheelchair-bound mute. Mark learns from sickly Countess Elise (Nicolodi) that Rose has disappeared. After the two find blood on the carpet outside Rose's room, Mark follows the stains. He becomes ill and falls unconscious. Elise sees a black-robed figure dragging Mark away, but the figure suddenly stops and gives chase to Elise. The figure eventually finds her swarmed with rabid cats in a room and stabs her to death. Mark staggers to the house's foyer where the nurse and caretaker (Valli) put him to bed.
Mark asks Kazanian (Sacha PitoÃ«ff), the antique dealer who sold Rose The Three Mothers, about Rose. However, the man provides no information. That night, Kazanian drowns several cats in a Central Park pond and falls into the water. Hundreds of rats from a nearby drain crawl over him, gnawing his flesh. A hot dog vendor hears Kazanian's cries and rushes over. The man kills Kazanian with a knife.
Carol, the caretaker, discovers the corpse of Elise's butler (Leopoldo Mastelloni) in the Countess' apartment. Shocked, she drops a lit candle which starts a fire. Attempting to put out the flames, she becomes entangled in burning draperies and falls from a window to her death. Mark uses a clue from Rose's letter to discover that beneath each floor is a secret crawl space. He follows hidden passages to a suite of rooms where he finds Professor Arnold who reveals, via a mechanical voice generator, that he is in fact Varelli. He tries to kill Mark with a hypodermic injection. During the struggle, Varelli's neck is caught in his vocal apparatus, choking him. Mark frees him, only to be told by the dying man, "Even now you are being watched." Mark follows a shadowy figure to a lavishly furnished chamber, where he finds Varelli's nurse. Laughing maniacally, she reveals to him that she is Mater Tenebrarum. She suddenly transforms into Death Personified. However, the fire that has consumed much of the building enables Mark to escape from the witch's den. Debris crashes down on the fiend, destroying her.

Question: Where does Marks clue come from?
Answer: Roses letter