A severed human head was found outside a court in the western German city of Bonn on Tuesday.
In the evening, passers-by reported the presence of a part of a human body in front of the court entrance, a police spokesman said.
"We concluded shortly thereafter that it was a human head," the spokesman said.
Subsequently, a stump, presumably from the same body, was found nearby on the banks of the Rhine, near the historic customs house.
A distance of about one kilometer separates the two sites where the body parts were found.
Police arrested a 38-year-old suspect nearby and said he was known to be involved in drug offences.
Police added that they believe the suspected perpetrator and the unidentified victim are homeless.