Dateline | November 09, 2012
>>> like this is what you would see in a movie. i still find it hard to believe that it happened.
>> a bright, young woman murdered.
>> i lost my best friend .
>> a husband stabbed.
>> she said, oh, my god, you're bleeding.
>> the tragic end of a twisted love triangle , and a strange game.
>> i'm not passing judgment, but when a game involves a blind fold and handcuffs.
>> you want to think it's got to be sexual.
>> two crimes, two mysteries that puzzled police until they found a link to one woman in love.
>> she had her nails done that morning. she spoke to her friends that morning. 8:00 comes, we're still calling.
>> we're waiting for you two. give me a call back as soon as you get this.
>> we must have called her phone 25 times, if not more that night, looking for annalisa.
>> so dinner went on without nelson and annalisa.
>> driving home from the city that night, i said we should stop by the condo to see her, and for some reason we didn't.
>> what christine and her friends could not have known was that someone else had gone to the condo earlier in the day.
>> hello? a guy is -- attacked my neighbor.
>> you think someone attacked your neighbor?
>> a 911 call had sent police rushing to anna lisa 's home just after noon. tom mcginty would lead the investigation with greg holt.
>> what did you find?
>> the door ajar. you saw the female victim lying on her back and the scene of a violent struggle.
>> reporter: it was annalesiona. she had been beaten and stabbed repeatedly, right in the middle of the day .
>> there were plants overturned, dirt on the floor, very bloody scene.
>> did it look like she had struggled?
>> very much so. fought for her life.
>> reporter: there was no sign of forced entry. the investigators figured annalisa had opened the door to her killer. was it someone she knew?
>> and who was on the short list of potential suspects here?
>> you always start with a small circle and go out. her small circle would have started with her boyfriend.
>> nelson sesler?
>> police didn't have to go looking for nelson. he drove up to annalisa's complex right about the time they were supposed to go to dinner. investigators broke the news to him and then told him to stick around.
>> he was asked to wait outside and he was put in an area of the condo which is commonly referred to as the boathouse.
>> reporter: about 700 miles away , local police in blaoomfield hills, michigan, were bringing the terrible news to her family.
>> how did you learn that your sister had been killed?
>> there was a loud knock on our front door. it shocked us out of bed. i went downstairs and, you know, there were police. it was hard to process that this had happened. i still kept thinking they couldn't have gotten it right.
>> reporter: bernadette had to tell her parents.
>> i said what, what happened to her? was she in an accident? i told her to be careful, you know. i was just shocked and crying.
>> i felt so bad and so weak and it was just shocking.
>> reporter: back in connecticut, stamford police were getting a surprisingly different vibe from annalisa's boyfriend, nelson.
>> was she emotional?
>> if he was, he didn't show it to us.
>> did he ask what happened to annalisa?
>> never to us.
>> no, not at all.
>> never to us. in fact went to sleep.
>> he went to sleep?
>> went to sleep in the boathouse.
>> as seasoned detectives, what did that say to you?
>> it raised a big flag with us. big red flag with us.
>> makes you wonder, did nelson know more than he was telling police about the murder of his girlfriend? and what would have been his motive for murder? no one could figure it out, especially his friends.
>> how could he do this? why would he do this? it didn't make sense.
>> neither did a strange game between a husband and wife, but it would provide a critical clue to solving the murder when "obsession" continues. [