Mike Schroepfer, Facebook's chief technology officer, was tearing up.

For half an hour, we had been sitting in a conference room at Facebook's headquarters, surrounded by whiteboards covered in blue and red marker, discussing the technical difficulties of removing toxic content from the social network. Then we brought up an episode where the challenges had proved insurmountable: the shootings in Christchurch.

In March, a gunman had killed 51 people in two mosques there and live streamed it on Facebook. It took the company roughly an hour to remove the video from its site. By then, the bloody footage had spread


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