Actually, it is both of them. In the movie, at the beginning of the rape scene of F. Alexander and his wife, ALex sings "Singing in the Rain." Later on, after F. Alexander takes him in his home and Alex gets in the shower, Mr. Alexander hears him singing "Singing in the Rain" and recognizes him as his wife's murderer.
In the book, he discovers Alex is the murderer when Alex accidentally reveals he knows much about Mr. Alexanders home than he should to have just arrived, such as remarking about how there is no phone (which he asks to use in the beginning of the book to gain access to the house.) SarahLeshea (talk) 22:42, 8 April 2009 (UTC)