gail to dr. robert bierenbaum. >> reporter: robert bierenbaum met gail during his surgical residency in new york. >> he's close to her age, jewish, classical guitar player. he spoke several languages. he was a gourmet cook. an expert skier. he had mastered instructor level piloting. >> reporter: the doctor swept gail off her feet immediately. >> however, in spite of all of his pluses, there were problems with the relationship. and it was obvious early on. >> when we would be together with them as a couple, he would be very controlling of her. >> reporter: but gail had made up her mind. bob was marrying material. and the two get engaged. but then all of a sudden those red flags about gail's seemingly perfect fiance become four-alarm sirens. >> during the summer of '82 i get the hysterical call from my sister. bob tried to kill the cat. he had the cat in the toilet choking it with its head submerged underwater. >> he was offended because she seemed to love the cat more than she loved bob. >> no, no, no, alayne, we're going to get rid of the cat and then everything's going to be fine because he's go

they've arrested robert bierenbaum for the murder of gail katz. i started screaming. >> dr. uld commit a murder. unless you saw his other side. my life went flashing in front of my eyes. >> he was moving all over. maybe to get as far away from new york as he could. >> after he left i'm confident he thought the ball game was over. >> i didn't want him to get away with it. i wanted every day of his life to be miserable. >> i said, were you ever married before? i thought, whoa, i touched a nerve. >> did he do it? could he have done it? >> could it have been me? >> the first thing that stood out to me were the flight records. oh, my god. there's the anxioexplanation whl not around anymore. >> i didn't want anybody to dig her up. >> literally, they're going to have to exhume the body they think is games. >> i am so devastated. >> what was this that grave? >> he has no idea what is coming down the road, and it's us. >>> vegas 19, 1990, we had dazzle. lots of glitz. lots of beautiful, beautiful showgirls. cocktail waitresses. >> plastic surgeons were starting to flock here. this was

it turned things upside-down. >> i get a phone call, they've arrested robert bierenbaum for the murder of gail >> since 2000, he has been behind bars serving that sentence of 20 to life insisting he was innocent. just last december, he was up for parole. >> reporter: "20/20" obtained this transcript of that parole hearing and in it is a shocking revelation. for the first time in 35 years bierenbaum finally admits to killing gail. >> the earth shifted. i was in shock. >> tonight on "20/20" much more from the murder victim's sister and never before heard shocking details from bob's last vegas girlfriend. >> wow a story. wow. john, thank you to you. >>> ginger, we'll head over to you. >> yes, and we have three confirmed tornadoes by wtae reporting that this morning, certainly damage around the pittsburgh area, thanks to a cold front that for the rest of us is just bringing a return to autumn. the temperatures will dip into the 40s again for new york and boston. raleigh, even, by sunday morning. sub 50. now i have to tell you that noaa put out their official winter outlook, and there's a strong si