we might be on to something. >> reporter: looking good until investigators did some testing on the blood. >> we did a presumptive out there, and it showed negative, back to square one. we're not dealing with blood. >> reporter: well, what about derek denying he knew danielle, even though friends claimed he called her his sister? didn't that make him a liar? one of derek's friends cautioned don't read too much into that. >> he tries to seem cool and tell everybody is his sister, everybody's his sister, everybody's his brother. that's just the kind of guy he is. >> reporter: detectives also spoke to one of derek's biggest defenders. his girlfriend. >> her information suggested derek could not possibly have been involved in danielle's disappearance. he had no way of getting to her house that night. >> does he have a car or anything? >> he doesn't even have his license. >> reporter: caroline knew derek too and thought he was a decent kid, misunderstood. >> i guess everyone thought that he was that type of person because he's -- he was the kid that skipped school and that didn't -- got in trouble with teachers, or spoke back, or something like that. >> school bad boy, huh?