Megan Hunt came up with a terrific idea to trap the killer. This went beyond DNA.
We also see loyalty among the cop's friends. They knew that he could not have done this.
Curtis does state that in cases involving the murder of a police officer, more is expected of those investigating the case.
The ending was the best part of this episode. A female drug user, who the officer was involved with, finds out that he did not lie to her and that he would help her in pursuing a singing career. Her singing at a club with all officers attending was great to view.