Vandals have damaged the entrance to a building in Hamburg that houses the offices of social network Facebook, smashing glass, throwing paint and spraying “Facebook dislike” on a wall, according to police in the northern German city.

Police said in a statement on Sunday that the overnight attack was carried out by a group of 15-20 people wearing black clothes and hoods. An investigation has been launched. Facebook was not immediately available to comment.

The European head of Facebook is under investigation in Germany over the social network’s alleged failure to remove racist hate speech. The investigation was announced last month as German politicians and celebrities expressed concern about the rise of xenophobic comments in German on Facebook and other social media sites as the country struggles to cope with the flow of refugees.

Facebook’s managing director for northern, central and eastern Europe, Martin Ott, who is based in Hamburg, may be held responsible for the social platform not removing hate speech, a spokeswoman for the prosecution said last month.

A Facebook spokesperson last month declined to comment on a possible investigation. “We can say that the allegations lack merit and there has been no violation of German law by Facebook or its employees.”

Facebook has a partnership with a group called FSM, which voluntarily monitors multimedia service providers, and has said it would encourage its users to push back against racism.