Douglas said a number of possible suspects have been ruled out through polygraph tests or because the suspect had an alibi. She pointed out that other possible suspects refuse to talk to police, who can't press further without more information to go on. The remaining active tips are reports from witnesses or information regarding the case that have to be verified. Often it turns out someone thinks they've seen something, but they can't be sure. Because of that, the trail is colder.
People forget things.
It's very frustrating as an investigator. Derrick is not considered a runaway. Other than her baffling disappearance, there's nothing to distinguish her behaviour that night from that of countless other young women who go out drinking with friends, and stay out late. Something has happened. We suspect foul play.