The body was found on Thursday at the Möhnesee lake some 60 kilometres from Paderborn. Bits of her clothing had been found in the area on Wednesday.
Police await autopsy results and have not identified the victim, but they fear it could be the body of 8-year-old Kardelen, whom they have been looking for since Monday.
Some 100 officers and several search dogs had been combing the area for clues since early morning and the body was found near a dam wall.
The 30-man special task force investigating the case plans to post signs on roads between Sauerland and Paderborn asking commuters who may have witnessed suspicious activity to contact them.