Obsession, Part 4

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>>> i did not set out to hurt him.

>> a woman named sheila davalloo stabs her husband during a weird guessing game and then of all people she calls nelson sessler, the boyfriend of a woman who had been stabbed to death just a few months before. co-worker and friend, christine dinerstein got the news right away.

>> i get to work. i go sit down at my desk. someone comes up to me and tells me did you hear what sheila davalloo did?

>> reporter: as it turns out, sheila davalloo also worked at purdue pharma . sheila was known as a competent biochemist and researcher, friendly, social, really smart.

>> she was very successful. a lot of promotions in the company.

>> reporter: the story about sheila and the knife swept through the office.

>> i've never heard of anyone getting stabbed in my life and here it is two stabbings in a short period of time. and they're both connected to nelson ? something does not make sense.

>> reporter: that's pretty much what detective carpentier was thinking. they asked nelson sessler to come back for another interview. investigator tom mcginty was there as well as sessler admitted that he and sheila davalloo were friendly.

>> he was talking about how they were such good friends and used to go skiing and she had purchased two dogs. he would go over to her house and walk them for her. finally i said, nelson , nobody is driving from connecticut to westchester and not getting laid. finally he admits that he was having a sexual relationship with sheila .

>> this was a bombshell. nelson was now admitting there had been another woman in his life. he had dated sheila and anna - lisa at the same time, but said he broke up with sheila as things got serious with anna - lisa . that was a few months before the murder, and after the murder, he and sheila hooked up again.

>> how did you react when sessler finally came clean to you and said yes, i was having a sexual relationship with sheila davalloo ?

>> i was very upset.

>> i mean he's been working this case, anna - lisa 's homicide day and night . the fact that nelson was keeping information from him was definitely upsetting to investigator mcginty.

>> nelson sessler said he had no idea sheila had a husband, and the husband didn't know anything about nelson , until detective carpentier told her. she also told paul about anna - lisa , and that's when it clicked. that name, anna - lisa .

>> sheila was always telling me a story about people at work.

>> it was an office love triangle . sheila said it involved her very, very good friend melissa, who was hooked on a guy who had a girlfriend named anna - lisa and sheila talked about it a lot.

>> she seemed to want to tell everybody about it. she told me, she told friends of ours.

>> sheila 's friend, tammy may, heard about the office triangle and heard about it all the time.

>> sheila would talk about her friend almost more than she talked about herself. almost to the point where she was obsessed with it. she would even talk about their sex life quite a bit.

>> reporter: when sheila 's affair with nelson sessler was uncovered, the fiction dissolved.

>> jack turned out to be nelson sessler and melissa was sheila .

>> reporter: so all of sheila 's stories were about herself and her apparent obsession with nelson sessler. maybe nelson thought it was a casual fling, but sheila sure didn't, and it was only days after anna - lisa 's murder that sheila moved in on nelson again.

>> according to nelson sessler, sheila began to bring him care packages a couple of weeks after anna - lisa died.

>> it became very obsessive. when i think nelson learned the extent of the obsession, it became alarming.

>> how would you best describe sheila davalloo ?

>> manipulative.

>> manipulative.

>> highly intelligent.

>> and very good liar.

>> reporter: so suddenly and for the first time, police investigating the murder of anna - lisa raymundo and a suspect with a motive, and they wondered if nelson sessler broke up with sheila to be with anna - lisa , how far would sheila go to get him back.

>> coming up, she was clearly an obsessed woman, but is anybody this cold-hearted?

>> she had her nails done that morning. she spoke to her friends that morning.

>> it's like she had a list. nails done, talk to friends, kill my husband.

>> and make sure nelson gets here by 8:30 so we can have dinner.

>> when "dateline" continues. people really