Apartment Next Door, The by William Johnston (1871 - 1929)

Jane Strong witnessed two men chasing each other in the middle of the night; then heard a gunshot. The man who came back was none other but the tenant next door. The next day, Jane reads that the man, who chased the tenant, had committed suicide. Now, she alone knows that it was a murder. The United States Government has recruited her to help them catch the murderer. The question is: Did he kill him? - Summary by SaraHale