In response to theory #4021 and other theories that mention Rebecca's escort and poisoning by Red John. I think it is important that we take into consideration , not only who could have had access in the prison, but the fact that they were about the same height as Rebecca. While two of the Sheriffs escort her you notice Rebecca does not look up at him coming down the hall, her eyes stay in a view where shes looking straight the whole time , Even as Red John gets closer , which is important because Rebecca is about 5,2" - 5,3" (just a estimate), and if somebody taller than 5 feet came in close view your head would automatically have to move up in order for you to be able to physically look the person in the eye. She did not do this so we all know who out of the suspects are about Rebecca's height here .. .. .. Its all in Season 2, episode 8.