How does Dr. Forrester determine that Glenn is responsible for Jenny's rape in the novel, All Is Not Forgotten?

When Forrester hears about Jenny's carving, he knows the rapist is Glenn. He pays Glenn a visit and plants the thought that he should commit suicide because the crime was not perfect. A little later, Glenn does just that. Before doing so, he prepares a written account of his rape of Jenny along with the mask and glove used in the crime. Forrester does not take his suspicion of Glenn to the police because he wants to help Jenny get her memory back so they can both share in the pain of being carved after being raped.