The emergency exercise, details of which were revealed in company documents leaked by whistleblower Frances Haugen, was internally described as a "watershed moment" for Facebook's live video service.
The attacker could broadcast the attack on the two mosques' lives for 17 minutes, but it was not detected by the company's systems, allowing it to be quickly replicated online. In the next 24 hours, Facebook deleted 1.5 million uploads.
"The training data set includes police/military body camera footage, entertainment shots, and simulations," as well as "military videos" obtained from the company's law enforcement outreach team, the internal document said. It also includes video clips from first-person shooters as examples of unblocked content.
As a result of these and other efforts, Facebook believes it has reduced detection times from five minutes to 12 seconds, the documents show. Christchurch's video now has a score of 0.96 for internally violent images, well above the intervention threshold.
Elsewhere, the set of leaked documents shows how eager Facebook is to repair its tarnished image. The company acknowledged that it had "taken minimal restrictions". In May 2019, the company announced a "one strike" policy, banning accounts from Using Live for 30 days for just one terrorist violation.
The change, announced to coincide with the Christchurch summit in Paris, is aimed at removing terrorist content from the web. New Zealand's prime minister, Jacinda Ardern, "used Facebook Live to update her fans after the announcement," which Facebook called "a major public relations win."