The Walking Dead Scoop: Who's the Mystery Killer?

As if a killer flu, killer zombies and the threat of a killer Governor roaming in the wind wasn't enough, now The Walking Dead's rag-tag group of survivors are facing the threat of an actual killer inside the prison walls.

The fourth season kicked off with a mysterious flu claiming one of the Woodbury transplants. But when Karen Karen (Melissa Ponzio) and another guy (let's just call him Red Shirt, shall we?) also died, it became clear the flu wasn't the cause. Instead, Tyreese (Chad Coleman) discovered their dead and burned bodies in the prison yard.

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So, who killed them? "There was someone or something inside the prison that is killing our own," Andrew Lincoln tells TVGuide.com. "There's a murderer there, and there's a retired cop that may come out of retirement to see if he can puzzle that out."

With Rick on the case, it won't be long before viewers find out the identity of the culprit. Could that person be whoever has been feeding the walkers rats? "That would be a safe assumption to make, but I can't really say," executive producer Robert Kirkman teases.

However, Karen's murder will be a turning point for Tyreese, who goes on to be Rick's right hand man in the comics. "Tyreese has been somewhat insulated," Kirkman says. "He's had Sasha [Sonequa Martin-Green] with him and he hasn't really lost any loved ones in this world. So, losing Karen in the way he did affects him in very dramatic ways. It can be an issue that really is going to alienate him from other people because he's lost the woman he loved and everyone else is like, 'We've been dealing with this for years now.' He's just now coming around to the sorrow that is inherent in this world, and it's something that is really going to change him quite a bit."

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But Tyreese's sorrow could also be the catalyst for his relationship with Rick to blossom. "These issues that have come up are going to bring these two together and they're going to have to work together to solve this," Kirkman says. "There's going to be a mutual respect between them like we see in the comic books."

Who do you think killed Karen and Red Shirt?

The Walking Dead airs Sundays at 9/8c on AMC.