If grisly images stay up on Facebook or YouTube long enough, self-appointed detectives around the world sometimes use them to reconstruct a crime scene. In July 2017, a video capturing the execution of 18 people appeared on Facebook. The clip opened with a half-dozen armed men presiding over several rows of detainees. Dressed in bright-orange jumpsuits and black hoods, the captives knelt in the gravel, hands tied behind their back. They never saw what was coming. The gunmen raised their weapons and fired, and the first row of victims crumpled to the earth. The executioners repeated this act four times, following the orders of a confident young man dressed in a black cap and camouflage trousers. If you slowed the video down frame by frame, you could see that his black T-shirt bore the logo of the Al-Saiqa Brigade, an elite unit of the Libyan National Army. That was clue No. 1: This happened in Libya.
Facebook took down the bloody video, whose source has yet to be conclusively determined, shortly after it surfaced. But it existed online long enough for copies to spread to other social-networking sites. Independently, human-rights activists, prosecutors, and other internet users in multiple countries scoured the clip for clues and soon established that the killings had occurred on the outskirts of Benghazi. The ringleader, these investigators concluded, was Mahmoud Mustafa Busayf al-Werfalli, an Al-Saiqa commander. Within a month, the International Criminal Court had charged Werfalli with the murder of 33 people in seven separate incidents—from June 2016 to the July 2017 killings that landed on Facebook. In the ICC arrest warrant, prosecutors relied heavily on digital evidence collected from social-media sites.