Chameleon

S4 E1: A routine vice raid at a gentleman's club uncovers an irate prostitute named Lisa (Sara Ramirez) who is pushy, annoying and insists she was raped by one of the club's patrons. The Special Victims Unit is called in and Lisa tells Benson (Mariska Hargitay) and Stabler (Christopher Meloni) that a man tried to kill her, moments later her fellow co-worker is found dead. It takes a while but the Detectives finally piece together enough information to get a sketch of their suspect. They trace him to the Palm Hotel where they arrive just in time to hear gunshots go off. They bust into the room to find a prostitute named Debbie (Sharon Lawrence), who has been beaten up, holding the gun that killed their guy, Jon Becker. They interview Debbie in the hospital, determine she was acting out of self-defense, and decide not to press charges. That is, until ballistics determines that the murder weapon had been used in an unsolved homicide six months before - Becker was in prison at the time, which means it could have been Debbie's gun.