according to deputy d.a. larry yellin. >> should be a college girl and worried about grades and boys and football games and those things. >> reporter: one wrong turn and you never know? >> yeah. >> reporter: but almost three weeks in, detective julissa trapp seemed stuck. >> i think she got a little frustrated and got a little desperate and came up with the idea of using the computer database. >> reporter: that is the computer database of sex offenders. if they had a serial killer on their hands, there was at least a chance he'd already run afoul of the law at some point. it was a bit like just poking a finger into the haystack frankly and hoping to encounter a needle. worth a try. so trapp called this woman. sexual assault detective laura la mella. >> all sex offenders on parole will have an anklet, a gps monitor. >> reporter: she asked where any