https://www.techdirt.com/articles/20200828/13525845201/content-moderation-case-study-facebook-responds-to-live-streamed-mass-shooting-march-2019.shtml

Summary: On March 15, 2019, the unimaginable happened. A Facebook user -- utilizing the platform's live-streaming option -- filmed himself shooting mosque attendees in Christchurch, New Zealand.

By the end of the shooting, the shooter had killed 51 people and injured 49. Only the first shooting was live-streamed, but Facebook was unable to end the stream before it had been viewed by a few hundred users and shared by a few thousand more.