police hone in on a mild-mannered orderly named donald harvey. under questioning, he admits to poisoning powell by pouring cyanide into his feeding tube. harvey calls it a mercy killing. the patient reminded him of his father. attorney william whalen was appointed to defend harvey. whalen thought he was dealing with a single mercy killing. but rumors of other murders began to circulate. >> i went to the justice center and sat down with donald. and i said have you killed more than one person? and i fully expected him to say no. and he answered yes. i asked him to tell me how many people he killed. he said i can't. i got irritated with him and said i just told you, you have to be honest with me. and he said you don't understand. i can only estimate. i told him to, in his mind, pick a number that he knew it could not possibly go beyond. he said 70. we talked for 45 minutes more. and i could never tell you what we talked about. i mean, my mind, all i could see was a red neon sign blinking the number 70. >> whalen offered the prosecutor's office a deal

. >> donald harvey, a hospital care giver with a deadly secret. >> do you consider yourself a society yo path. >> that's what most consider me. >> what do you think? >> i have been rehabilitated. i'm ready to go back to the streets. >>> i'm john douglas. for close to 25 years i ran the fbi's behavior science unit. as a criminal from filer my job so is to help criminal investigators across the country looking for signs to help point the way to the killer. the only way to understand the criminal mind is to get inside