In the agency’s latest grand challenge, teams competed for $2 million and a chance to shape the future of communication technology by finding a better way to carve up the radio spectrum.

The context: The vast majority of Facebook’s moderation is now done automatically by the company’s machine-learning systems, reducing the amount of harrowing content its moderators have to review. In…

So, what are we seeing here?: The company has been training its machine-learning systems to identify and label objects in videos—from the mundane, such as vases or people—to the dangerous, such as guns or knives. Facebook’s AI uses two main approaches to look for dangerous content. One is to employ neural networks that look for features and behaviors of known objects and label them with varying percentages of confidence (as we can see in the video, above.)

Training in progress: These neural networks are trained on a combination of pre-labelled videos from its human reviewers, reports from users, and soon, from videos taken by London’s Metropolitan Police. The neural nets are able to use this information to guess what the entire scene might be showing, and whether it contains any behavior or images that should be flagged. It gave more details on how its systems work at a press briefing this week.

Then what?: If the system decides that a video file contains problematic images or behavior, it can remove it automatically or send it to a human content reviewer. If it breaks the rules, Facebook can then create a hash—a unique string of numbers—to denote it and propagate that throughout the system so that other matching content will be automatically deleted if someone tries to re-upload it. These content violating hashes can be shared with other social media firms so they can also take down copies of the offending file.

“These [Metropolitan Police] videos are incredibly useful for us. Terrorist events are rare, thankfully, but it means the amount of training data is so small,” engineering manager Nicola Bortignon said on a call.

remains one of the field’s biggest challenges.

The bigger picture: In March, a terrorist killed 49 people at two mosques in Christchurch, New Zealand. He livestreamed the massacre on Facebook, and videos of it circulated around the site for months afterwards. It was a wake-up call for the industry. If it happened again now, there is a better chance it would be caught and removed more quickly.