. >> reporter: detective jodi gonterman has been with the albuquerque police department for 17 years >> the first moment i met jodie, she told me, "i'm going to find the person who did this." >> i kind of have a little ocd. if i have something that needs to be done or finished, i have to complete it. >> reporter: and right off the bat, she has an unusual suggestion. something her department had never tried before. >> i wasn't so convinced that her loss of memory was due to brain damage. i thought it could have been that she was suppressing the traumatic memories that she had. she said, let's try something new. >> reporter: hypnosis. meet forensic psychologist dr. leon morris. he hypnotizes some patients to unlock repressed traumatic memories. that was literally the first time you met her. >> absolutely. all of my contact with her is on the video. tell me why you're here today. >> let's see. there was an attack about almost six years ago. brutally beaten on the left side of my head. do i remember any of that? simply no. >> reporter: you didn't know in the beginning whether or not her

. >> reporter: that name leading detective jodi gonterman to this man, a doting father of four with no the mall. >> when is the last time you saw brittany in relation to when the attack happened? >> probably at the mall. i have seen her at the mall. >> reporter: detectives get hansen's dna after a discarded fast food cup and test it against that drop of blood found all those years ago. >> she handed me a folder. i opened it up and it was justin hansen's photo and she put match jumped up. i started screaming. >> reporter: hansen is arrested and charged but denies involvement. >> you did this to her. >> i didn't do that to her. >> reporter: today brittany and her mom are convinced it is him. >> memory is really fluid. are you certain it's him? >> i'm 100% positive it's him. >> coming up tonight on "20/20" we'll hear about the other evidence or lack of evidence that justin hansen says proves he did not do it including fingerprints and other dna found at the scene. investigators say simply cannot be his. we'll also hear from justin hansen himself. >> and brittany's mother said she saw the