Billy Schmidt, an eccentric man who lived across the hall from Sister Cathy's apartment, was accused of the murder years later by his former sister-in-law. According to this sister, Billy dated a man only known as "Skippy," who frightened her and fanned the flame of Billy's peculiarities and instabilities and may have gotten both Billy and her husband involved in something violent.
When her husband arrived home covered in blood one night around the time of the disappearance without decent explanation and then descended into an alcoholic spiral, she immediately began to suspect that he and his brother were involved in the killing.
Billy's brother cited the fact that "we killed a girl and buried her behind the shop" as the reason for his drinking, and Billy had something of a mental breakdown following the event. (Read: keeping a nun's habit on a mannequin and babbling about "the woman in the attic.") Billy committed suicide months after the murder.
Add the fact that Billy was a known smoker of a specific cigarette brand that matched a butt found near Cathy's body, and it seems possible that he could be involved with Cathy's death.
However, internet sleuths take issue with several parts of the story involving Billy. Namely, the fact that Billy's sister-in-law wasn't concerned with her own husband who showed up covered in blood on the night of the disappearance:
In addition, Billy (who was gay) was not involved in the Catholic church and therefore had no reason to participate in a cover-up plot:
