A shadowy website spent hundreds of thousands of dollars on political adverts on Facebook and other networks. But the people behind “Mainstream Network” have managed to remain entirely anonymous. No-one knows who runs the site and its social media accounts, where their money is coming from, or what their motivation is. We investigate their operation - just as Facebook faces tough questions about their role in preserving the integrity of elections around the world.

Presenter: Mike Wendling

(Photo: A montage of Mainstream Network’s website and the site’s logo. Credit: Mainstream Network/BBC)