john lewin had always figured the alibi marshall got from his old girlfriend, yvonne hargrove, was a . what if detectives interviewed both yvonne and marshall on the very same day? would they trip themselves up with divergent stories? >> two teams went out the day that mr. marshall was contacted, one team contacted him, and a second team went to ohio and contacted her. >> yvonne was living in cleveland by then. >> and they say we need to talk to you about something that happened 20 years ago. and my brain went -- and i'm like, okay. so they tell me they're reopening up the one about william and the kfc. they asked me was he with you? i said, yes, the whole night. >> all those years later, yvonne insisted the story she told then and now was absolutely positively true. william marshall's alibi remained rock solid. meanwhile, 2,500 miles away, high in the mountains, detectives pulled up to william marshall's house not knowing what to expect. >> for whatever reason, he was comfortable enough with us to let us in. >> things unfolded slowly. marshall was cooperative and even keeled though i