<Tr> <Td_colspan="2"> The Walking Dead (season 3) List of The Walking Dead episodes </Td> </Tr> <P> "Killer Within" is the fourth episode of the third season and 23rd episode overall of the post-apocalyptic horror television series The Walking Dead . It originally aired on AMC in the United States on November 4, 2012, and was written by Sang Kyu Kim and directed by Guy Ferland . The episode marked Sarah Wayne Callies and IronE Singleton's final appearances as Lori Grimes and Theodore "T - Dog" Douglas and as regular actors on the show . Most television critics positively reviewed the episode, applauding its bleak undertones and the performances . However, some commentators criticized the narrative ploys and the direction taken in the program . "Killer Within" was viewed by 9.27 million viewers, including 4.9 percent of those in the 18 to 49 demographic upon its initial broadcast in the United States . </P> <P> Outside the prison, an unidentified individual lures a group of walkers towards the prison, breaking open the front gate's lock to allow them inside . </P> <P> In Woodbury, Michonne (Danai Gurira) is suspicious of how The Governor (David Morrissey) recovered supplies from a nearby National Guard camp, but he refuses to discuss it . She expresses her concerns with Andrea (Laurie Holden), and believes they should leave and head for the coast . Andrea would rather stay, and provides information on the Greene farmstead to allow Merle (Michael Rooker) to look for his brother Daryl . The Governor tells Meryl he will allow the search once Meryl obtains more concrete information . Andrea begins to become romantically involved with The Governor . </P>

Who let the walkers in the prison in season 3