Emergency services recovered a male body from the Braake in Brunsbüttel on Sunday.
A violent crime will not be ruled out, said chief prosecutor Peter Müller-Rakow on Monday.
The man's identity has now been clarified, and an autopsy of the body should clarify the cause of death.
The media had previously reported on the find.
Brunsbüttel - A woman had previously been discovered by the river, she is being treated in a hospital and has not yet been interviewed.
"It seems that there was a personal relationship between the two," said Müller-Rakow.
As the "Bild" newspaper and the "Norddeutsche Rundschau" reported, the woman was discovered unconscious on the bank of the Braake an hour and a half before the body was found.
Müller-Rakow did not provide any information on this.
dpa
