And he attacked the way that Facebook users are pushed towards other partisan political groups.
"Facebook's system of recommending political groups poses grave threats to American democracy and public safety," wrote the lawmaker from Massachusetts.
"Many Facebook groups (pages on your platform that function as forums for users to communicate about a particular topic) are breeding grounds for hate, echo-chambers of misinformation, and venues for coordination of violence, including explicit planning for the insurrection at the US Capitol on 6 January 2021.”
Mr Markey also said that the company had "failed to meet commitments it made" before the Senate to stop the practice of recommendations in groups for political content and social group issues.
The pro-Trump rioters openly used social media platforms, including Facebook, to organise the attacks on the Capitol following Donald Trump’s “Stop the Steal” rally on 6 January.
Mr Markey’s office also pointed to an investigation by The Markup that 12 of the top 100 groups being recommended on Facebook are still political, including violent ones.
He is demanding the company provides an explanation by 9 February.
"We have a clear policy against recommending civic or political groups on our platforms, and are investigating why these were recommended in the first place," said Facebook spokesperson Daniel Roberts.