in death, they thought kathleen peterson and elizabeth ratliff could have been twins. >> i'm just thinkingetting a whole lot better. >> reporter: kathleen's sister candace thought that peterson had killed both women. >> i have a better chance of being struck by lightning than finding two people who i intimately know at the bottom of a staircase. >> reporter: but to martha and margaret, the whole thing seemed absurd. the fact that michael was being accused of killing kathleen, the woman they called mom, was bizarre enough. but now, their birth mom, too? what would their father have gained by killing elizabeth ratliff? >> he would've gotten two screaming little ragamuffin kids out of it and that's it? like, there's nothing -- there's no reason for it. >> reporter: for the investigators in north carolina, though, the death in germany became a strong building block in their circumstantial case for murder. and what's more -- detectives learned that michael peterson had a secret life. secrets -- tawdry ones -- were about to spill out in the durham courthouse. >>> coming up -- enter brad, the mal

the future of awesome. >>> continuing now with dramatic new developments in the case of kathleen petersonnot look like a fall. >> prosecutors have charged kathleen's husband michael with murder, telling the jury about his connection to another suspicious stairway death 20 years earlier in germany. >> do you really believe that lightning strikes twice in the same place? do you? >> now, michael peterson sits down with us to refute the prosecution's case. including that other supposedly damn death. >> they said, my god, there was blood everywhere. well, no. the german police didn't see any blood. the american military didn't see any blood. >> and then a clash outside the courtroom. >> pleading guilty -- >> actually, we're not -- >> you're pleading guilty. >> here again, dennis murphy. >> reporter: the state's case lasted more than two months. and each day, michael peterson's girls, margaret and martha, sat in court, suffering as prosecutors labeled their dad a killer. >> they would accuse my father of double murders, or the wife murder, or the staircase murders? >> and we couldn't stand up a