The Killing/WMG

Gwen is the killer

 * There's something wrong about that woman. She clearly has ice in her veins. I could easily see her killing some random girl just to create fuel for her candidate's campaign.

Terry, the aunt, killed her.

 * No one's really looked at her much as a suspect, and her mother didn't seem to pay much attention to her compared to Mitch.
 * During the first season, she was the best suspect based on Roger Ebert's theory of "Conservation of Characters," by which, a TV show or movie can't pay a lost of actors who aren't necessary to the plot, so the person who just seems randomly stuck in, neither moving the plot forward, nor performing an obvious function, like the medical examiner, is usually there for the purpose of being revealed as the killer at the end. However, now that Terry is functioning in Mitch's role, Mitch begins to seem like the redundant character....
 * Regarding "Conservation of Characters": anytime the police call in two experts, someone has two children, and they seem interchangeable, or there are just unnecessarily two people doing something one person could do, you have to wonder. It made the older of Rosie's two brothers suspicious, but he really is too young to put her body in a trunk, steal a car and not get caught, get into a casino, and soforth.
 * Jack, Linden's son always seemed unnecessary and extraneous to the plot, even in the way. He had no function at all, until he explained the Manga character.

Rick, Linden's fiancee, did it.

 * He wanted to test his future wife so he killed a random girl just to find out if she would choose him over the investigation.

Holder is being blackmailed.

 * Someone who knows that Holder was on drugs is the killer, and is blackmailing him into helping them.