Carnival of Souls

Mary Henry (Candace Hilligoss) is riding in a car with two other young women when some men challenge them to a drag race. As they speed across a bridge, the women's car plunges over the side into the river. The police spend three hours dragging the murky, fast-running water without success. Mary miraculously surfaces, but she cannot remember how she survived.
Mary then drives to Utah, where she has been hired as a church organist. At one point, she can get nothing on her car radio but strange organ music. She passes a large, abandoned pavilion sitting all by itself on the shores of the Great Salt Lake; it seems to beckon to her in the twilight. Shortly thereafter, while she is speeding along a deserted stretch of road, a ghoulish, pasty-faced figure (never identified, but called "The Man" in dialogue and played by director Herk Harvey, uncredited) replaces her reflection in the passenger window and stares at her. When The Man suddenly appears in front of her, she swerves off the road. At a gas station, the attendant tells her the pavilion was first a bathhouse, then a dance hall, and finally a carnival before shutting down.
In town, Mary rents a room from Mrs. Thomas; John Linden, the only other lodger, wants to become better acquainted with the blonde newcomer, but she is not interested. That night, she becomes upset when she sees The Man downstairs in the large house and retreats to her room. Mrs. Thomas, who brings her some food, says she did not pass anyone.
Soon, Mary begins experiencing terrifying interludes when she becomes invisible and inaudible to the rest of the world, as if she simply is not there. When The Man appears briefly in front of her in a park, she flees, right into the arms of a Dr. Samuels. He tries to help her, even as he acknowledges he is not a psychiatrist.
Her new employer, the minister (Art Ellison), is put off when she declines his suggestion of a reception to meet the congregation. When she practices for the first time, she finds herself shifting from a hymn to eerie music. In a trance, she sees The Man and others of his ilk dancing at the pavilion. The minister, hearing the strange music, denounces it as "profane" and insists upon her resignation.
Terrified of being alone, Mary agrees to go out on a date with Linden. When they return home, he smooth-talks his way into her room, but when she sees The Man in the mirror, she becomes upset and tries to tell Linden what has been happening to her. He leaves, believing she is losing her mind.
After going back to visit Samuels' office, Mary believes she has to go to the pavilion. There, she is attacked by The Man and his fellow ghouls. Mary tries frantically to escape, at one point boarding a bus to leave town, only to find that all the passengers are ghouls. Then she wakes up, showing that she dreamed this sequence at least. In the end, she is drawn back to the pavilion, where she finds her tormenters dancing. A pale version of herself is paired with The Man. When she runs away, they chase her out onto the beach. She collapses, and they close in.
The minister, the doctor and the police go to the pavilion to look for her. They find her footprints in the sand - the only ones - but they end abruptly, and there is no other trace of her. Finally, the car is located and pulled from the river. Mary's body is in the front seat alongside those of the other two girls.

Question: Who does Mary find out that all the passengers in the bus are?
Answer: Ghouls