Stranger on the Third Floor

Stranger on the Third Floor is a 1940 film noir, starring Peter Lorre and released by RKO Radio Pictures. The film was directed by Boris Ingster and written by Frank Partos with Nathanael West uncredited.

Reporter Michael Ward is the key witness in a murder trial. His evidence that he saw the accused Briggs standing over the body of a man in a diner is instrumental in having Briggs found guilty.Afterwards Wards fiancee Jane is worried whether Ward was correct in what he saw and Ward becomes haunted by this question. Next, Wards neighbor is killed the same way as the man in the diner, but Ward is arrested for trying to point this out to the police. As a result, Jane goes out to try to clear Ward by finding the sinister stranger that Ward saw on the stairwell. ........

Source: Wikipedia