Vandals have damaged more than 70 artworks and artefacts at some of Berlin's most renowned museums, police said, in a targeted attack kept quiet by authorities for more than two weeks.
Valuable paintings, stone sculptures and sarcophagi at three institutions on the German capital's UNESCO-listed Museum Island were sprayed with an "oily liquid", leaving visible stains, Berlin police told AFP.
German media called it "one of the biggest attacks on art and antiquities in German post-war history".
"The state criminal investigation office of the Berlin police has opened a probe over property damage to artworks and artefacts on display," spokesman Martin Dams said in a statement.
Mr Dams said police believe the vandalism occurred on 3 October, Germany Unity Day, during opening hours at the Pergamon Museum, Neues Museum and Alte Nationalgalerie.
He did not say why neither the museums nor the police had communicated earlier about the attack, which was first reported late yesterday in German media.
Mr Dams did not provide any information about a possible motive.
However, a report by Die Zeit weekly and public broadcaster Deutschlandfunk noted that Attila Hildmann, an activist who has railed against government measures to contain the coronavirus, had in August and September spread outlandish conspiracy theories about the Museum Island.
Using his Telegram channel, Mr Hildmann claimed the Pergamon Museum, closed for part of the summer due to the pandemic, held the "throne of Satan".
He said the institution was the centre of a "global satanist and corona criminal scene" where "they sacrifice humans at night and abuse children", in an echo of the international QAnon conspiracy movement.
Berlin daily Tagesspiegel said that museum visitors who had booked tickets for 3 October had been contacted by police to ask for help with the investigation.
A UNESCO World Heritage site, Berlin's Museum Island is home to precious artefacts, including a legendary bust of the Egyptian Queen Nefertiti and Babylon's Ishtar Gate.
It attracts around three million visitors each year and is undergoing a major renovation and expansion.