>> yes. >> reporter: detective gonterman remains undeterred by these inconsistencies.son who attacked brittani if only she could get it. >> we had to find it. i met with some of our undercover detectives, and i asked them, "can you follow this guy and get his dna?" >> he was at a fast food restaurant drinking from a cup, and he threw that cup into the trash can. those undercover officers then obtained that trash, including that cup, took it to the crime lab where it was analyzed. >> reporter: after one agonizing month, detective gonterman walks into a meeting with the crime lab analyst. >> she handed me a folder, and i opened it up, and it was the parabon snapshot report, it was justin hansen's photo, and she put match on it. i jumped up, i started screaming. that turned into crying. i was so happy, i just couldn't believe it. >> reporter: when we come back with the dna match in hand, it looks case closed. but not so fast. >> there was unknown dna evidence that actually excluded justin hansen. we knew that that would be a problem. >> reporter: so would the charges agai

. >> 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