Dutch police and the Public Prosecution Service have concluded their investigation into the disappearance of Tanja Groen, but the questions of what happened to the then-18-year-old and where she is remain unanswered, authorities said Monday. Groen vanished in the province of Limburg during the night of Aug. 31 to Sept. 1, 1993, after a party in Maastricht.In the 33 years since, investigators pursued countless tips, examined many DNA profiles and other traces, and carried out several large-scale searches. Those included opening a grave and digging up a large area of heathland.