The chief executives of Facebook, Google and Twitter faced criticism from US lawmakers on Thursday about their approaches to extremism and misinformation in their first appearances before Congress since pro-Trump rioters assaulted the US Capitol on Jan. 6.

“We fled as a mob desecrated the Capitol, the House floor, and our democratic process,” said Democratic Representative Mike Doyle. “That attack, and the movement that motivated it, started and was nourished on your platforms,” he added.

Facebook Inc Chief Executive Mark Zuckerberg said there was content related to the riot on its platforms but, when asked if the company bore responsibility for the event, he said the company’s responsibility was to “build effective systems.”

“We did our part to secure the integrity of the election, and then on Jan. 6, President Trump gave a speech rejecting the results and calling on people to fight,” he said. He argued that polarization in the country was due to the political and media environment.