“We have opened a larceny probe,” a spokesman for the Hamburg police said on Tuesday.

It was not immediately clear how and when the cranium vanished but staff at the Hamburg History Museum reported it missing on January 9.

The skull impaled on a large rusty nail was discovered in 1878 during construction for a warehouse district in an area where pirates had earlier been beheaded and their heads displayed on spikes as a grisly warning. The museum had long displayed the cranium, which was already missing a jawbone, as belonging to Klaus Störtebeker, who is believed to have been executed in 1401 with 30 henchmen outside the walls of the Hanseatic League city.

Later forensic analysis determined that the skull may well belong to a man beheaded around 1400, although not necessarily Störtebeker. The museum tried in vain in 2004 to produce a definitive link to Störtebeker with a DNA analysis comparing genetic material from the cranium with that of possible descendants.

Störtebeker, old German for “Tip Up the Mug,” earned his name for his fabled carousing. After a lengthy reign of terror on the North Seas, he was captured off the Helgoland archipelago and taken to Hamburg to be executed.