More than 60 artworks and artefacts at some of Berlin's best-known museums have been smeared with an oily liquid by an unknown perpetrator or perpetrators earlier this month, authorities say.
They were hopeful that the apparently random damage can be repaired but said the motive was a mystery.
The works at the Museum Island complex, a UNESCO world heritage site in the heart of the German capital that is one of the city's main tourist attractions, were targeted at some point between 10am and 6pm on October 3.
Investigators said they had watched hours of surveillance camera footage but hadn't found any obvious sign of anyone applying the liquid.
In all, 63 works at the Pergamon Museum, the Alte Nationalgalerie and the Neues Museum were affected, said Christina Haak, the deputy director of Berlin's state museums.
There was no thematic link between the targeted works and "no pattern is discernible" to the perpetrator's approach, Haak added.
The liquid was oily but not corrosive, said Friederike Seyfried, the director of Berlin's Egyptian collection, which is housed in the Neues Museum.
She wouldn't give more specific details of the colourless fluid, citing the ongoing investigation.
Carsten Pfohl, a senior official with Berlin's criminal police office, said that more than 3000 people visited the Museum Island on October 3, a Saturday on which Germany marked the 30th anniversary of its reunification.
Complicating investigators' efforts, most of that day's tickets were sold on site and only 1400 personalised tickets had been booked in advance; all who ordered the latter have been contacted by email to ask whether they noticed anything untoward.
Police said they had decided initially not to go public about the incident out of "tactical considerations related to the investigation".
On Tuesday night, the weekly Die Zeit and Deutschlandfunk radio broke the story.
On Wednesday, police called for witnesses to come forward with any accounts of suspicious people or events they noticed on October 3.
It wasn't clear how the liquid was applied to the works, Pfohl said.
Pfohl said that the incident isn't unique because artefacts in museums in other countries have been attacked with liquids over recent years.
Officials weren't aware of any threats.
The damage was discovered by museum staff.
Seyfried said the works affected didn't include any paintings and also weren't among the complex's best-known attractions.
Those attractions include treasures such as the Pergamon Altar, Babylon's Ishtar Gate and a famous bust of Egyptian Queen Nefertiti.
Australian Associated Press
