but in this instance, clint van zandt was the agent in the unit who got assigned to the heidi allen case in upstate new york. his take -- look for someone with a history of violence and someone obsessed with the case. >> the person who committed this is somebody who was really interested. i mean, the community was interested in the case, but this is more interest. >> reporter: obsessed. >> could not let it go. this is somebody who'll be saving newspaper articles. >> reporter: and there would be many newspaper articles, decades of them, in fact. the case of heidi allen was just getting started. >>> when we come back, clues to the mystery. >> who would grab an 18-year-old girl? >> our theory then was probably a sex crime. >> and then, unbelievably, the killer seems to out himself. >> he says, yeah, i killed this girl. >> he gives it up. >> he gives it up. for three generations. mm. heaven. real ingredients. from a real place. that's how we're working to bring out the best. and now, try new hellmann's organic for an irresistibly creamy taste. late friday and early saturday... and take 50 to