Bolsonaro’s supporters used social media to spread content during key events related to the Brasília attack. An unprecedented analysis of more than 15,000 public WhatsApp groups showed that these political actors tried to manufacture consensus in preparation for and after the attack. A cross-platform time series analysis showed that the spread of content on Twitter predicted the spread of content on WhatsApp. Twitter also predicted news coverage of Bolsonaro’s supporters, suggesting a propaganda feedback loop. Our findings indicate that investigative journalism and public policy initiatives could benefit from monitoring public groups on encrypted messaging apps.