‘The Walking Dead’ Preview: Who Set Loose the “Killer Within”?

Three episodes into the third season of AMC's 'The Walking Dead,' and it seems like we have a good bead on the major players and locations. Last week's "Walk With Me" introduced us to the town of Woodbury and its villainous (yet charming) Governor, while this Sunday's "Killer Within" will take us back within the confines of the prison with Rick Grimes and the other survivors. The question remains, however, is danger still afoot? What could cause Rick's group to come under undead attack yet again? Find out in the latest preview!

There is no safety behind bars, not even in 'The Walking Dead.' This Sunday's all-new episode "Killer Within" will focus mainly on Rick and the other prison survivors, as a mysterious accident unleashes hordes of walkers on the group from within the prison, as evidenced by the latest photos from The Hollywood Reporter!

But who could have been so careless as to let the zombies roam free throughout the prison, when Rick and the others took such great pains to clear out the facility in the season premiere? Has the Governor already found a means to hurt the survivors, or is there a threat we don't yet know about?

Check out the new photos from Sunday's 'The Walking Dead' "Killer Within," and give us your predictions for the episode in the comments!