Though we find the idea of Gerry Koob — Sister Cathy's secret romantic interest and close friend within the church — as the killer unconvincing, his reliability is called into question by many fans seeking justice.
Koob, who appears many times in the docuseries as a resource on the church, was the first person Cathy's roommate called to report her as missing. According to Koob, he, his friend, and Cathy's roommate prayed and took communion while awaiting the nun's return; however, she never returned home.
It is later revealed that Koob proposed to Cathy, suggesting that they both leave their roles (and subsequent celibacy vows) within the church in order to marry. According to Koob, Cathy declined his proposal.
Doubt about Koob's authenticity is questioned by several figures on the show, who believe his emphasis on an alibi (dinner and a movie with his friend) and particular details of his testimony don't add up. Koob also claims that Cathy never divulged her knowledge of abuse at the high school to him, which strikes some as strange considering that he supposedly proposed to her.
Toward the end of the series, a letter from Cathy to Koob is referenced, and the contents of the letter are far more romantic and sexually intimate than Koob initially let on about the nature of their relationship. This letter discusses menstruation, Cathy wanting to have Koob's children, and more. Regarding this letter, amateur sleuths have questioned whether Koob lied to preserve Cathy's reputation, or if there was something more nefarious going on. Some believe that the letter was not written by Cathy at all.
Another point of contention is Koob's insistence that a police officer presented him with Cathy's "vagina" wrapped in newspaper; the logistics of this claim confused many and tarnished their idea of the man as an honest, caring friend of Cathy's.
More than any other suspect, though, there remains a huge amount of support for Gerry Koob. Entire Facebook threads discuss his innocence, and many believe that even naming him as a potential killer is disrespectful to his obvious grief.
