"20/20" investigates the disappearances of two female coeds – both from close, loving families with everything to live for – who mysteriously vanished without a trace. Brooke Wilberger, 19, disappeared in broad daylight from a parking lot in the university town of Corvalis, Oregon in May of 2004. Maura Murray was a nursing student at the University of Massachusetts who, in February 2004, for an unknown reason, drove to New Hampshire, where her car was found crashed on the side of a desolate road. She has not been seen since. FBI statistics say there are more than 21,500 active missing persons cases involving people ages 18 to 29. As Elizabeth Vargas reports, how authorities respond to these cases can vary dramatically depending on the circumstances. Sometimes the police wait to react, since the missing person is legally an adult, as they did in the Murray case. But in Wilberger's case, the police did act immediately. What are the clues they're looking for? In one of these cases, police say they have an answer. Friday at 10 p.m. on Channel 9.