More than 60 artworks and artefacts at some of Berlin's best-known museums were attacked, smeared with an oily liquid by an unknown attacker or attackers. The motive remains unknown. 

The series of attacks that occurred on October 3 were announced by authorities on Wednesday. Investigators said they had watched hours of surveillance camera footage but hadn't found any obvious sign of anyone applying the liquid.

In total, 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 works which were all exhibited on the city's Museum Island complex, a UNESCO world heritage site, had been smeared with an oily, colorless but not corrosive liquid, said Friederike Seyfried, the director of Berlin's Egyptian collection.  

Carsten Pfohl, a senior official with Berlin's criminal police office, said that more than 3,000 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 1,400 personalized 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." But on Tuesday night, the weekly Die Zeit newspaper and Deutschlandfunk radio broke the story, describing the incident as one of Germany's most serious attacks on artworks in decades. 

Pfohl said police are investigating "in every direction," but wouldn't participate in media speculation that conspiracy theorists might be involved.