The young Turkish girl, identified by authorities only as Kardelen, was reported missing in the German state of North Rhine-Westphalia on Monday. Police said she was apparently sexually assaulted before being suffocated.
"The worst fears have been confirmed," said public prosecutor Ralph Vetter at a press conference on Friday afternoon, adding investigators had found an "important clue" that could help them catch the girl's murderer at the crime scene.
Some 100 officers and several search dogs had been combing the area for clues since early Thursday morning and the body was found near a dam wall covered by.
The special task force investigating the case has been increased to 50 officers. The police have posted signs on roads between Sauerland and Paderborn asking commuters who may have witnessed suspicious activity to contact them. But they refused to confirm reports on Friday that someone saw a man and child in the area where the girl's body was found.