Robert Mueller, the Special Counsel appointed to investigate possible Russian links to the Trump campaign in 2016, last week issued formal charges against three companies and thirteen named individuals. They are all Russian.

The 37-page indictment provides a wealth of detail on the scale and ambition of the operation, in which ordinary Americans were manipulated into not only promoting their messages online but even organising political rallies.

President Trump responded to the indictment by repeating his assertion that he had never colluded with the Russians.

It's not clear that those indicted will ever appear in an American courtroom, but if the allegations are true, they represent unprecedented interference in the American political system and raise questions about future vulnerability.

CONTRIBUTORS

Gordon Corera, BBC security correspondent

Molly McKew, expert in information warfare specialising in US-Russia relations, New Media Frontier

Andrei Soshnikov, BBC Moscow

Asha Rangappa, senior lecturer at the Jackson Institute for Global Affairs, Yale University and former FBI special agent, counterintelligence division.