Security is tight at this brick building on the western edge of Berlin. Inside, a sign warns: "Everybody without a badge is a potential spy!"
Spread over five floors, hundreds of men and women sit in rows of six scanning their computer screens. All have signed nondisclosure agreements. Four trauma specialists are at their disposal seven days a week.
They are the agents of Facebook. And they have the power to decide what is free speech and what is hate speech.
This is a deletion center, one of Facebook's largest, with more than 1,200 content moderators.
Germany, home to a tough new online hate speech law, has become a laboratory for one of the most pressing issues for governments today: how and whether to regulate the world's biggest social network.