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. arted screaming. >> so shocked i dropped to my knees. >> you must have been shocked. >> i was, shocked. >> gail bierenbaum vanished from the apartment she shared with her husband robert, a manhattan plastic surgeon. >> bob tells police that they had an argument and she came here to the park. >> yes. >> never in a million years you'd think he's be using cold, calculated murder in the same description of dr. bierenbaum. no way. >> no body. >> no forensics, no eye witnesses. >> the investigative theory was that he wrapped the body up, drove it to his plane in new jersey, flew the plane over the ocean -- >> you know, they've charged that you took her in an airplane and threw her out. >> yes, no comment. >> disbelief sort of switched to, good good, this guy was a psychopath. >> my god. you got to think it's a movie. >> crime and punishment. >> that's the story. >> that's stuff you see on tv. that's not real. right? >> the bierenbaum story, when you get down to it, is dr. jekyll and mr. hyde

it turned things upside-down. >> i get a phone call, they've arrested robert bierenbaum for the murderrenbaum. >> 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 s