Their probe will look at whether lawmakers knowingly or unknowingly helped the pro-Trump mob who tried to prevent the certification of Joe Biden’s election win, officials told CNN.
Phone data has indicated both communications between riot suspects and lawmakers on and around 6 January, as well as communications between rioters discussing their connections with members of Congress, the official said.
The findings did not necessarily show wrongdoing by lawmakers, the official noted, and the investigation is not directly targeting members of Congress.
The Justice Department now has more than two dozen prosecutors investigating the funding of the rioters and the role, if any, of lawmakers, the official said.
Law enforcement officials have sought cell phone tower information to identify who was at the Capitol during the violence.
Using an “exclusion list” investigators have been able to sift out those phones authorised to be in the Capitol, such as members of Congress, staff, law enforcement and government officials.
The move was necessary as US Capitol police let go hundreds of rioters arrested inside the building, and around 300 people are facing charges resulting from the violence.
Democrats have called for Capitol Police security footage to be examined to see if any lawmakers gave tours of the Capitol to rioters ahead of 6 January.
Senators Mike Lee and Josh Hawley questioned FBI Director Christopher Wray earlier this week on what investigators were doing with communications and financial data.
Mr Wray told Senators that he could not give them specifics about how the data was being used.
“I feel confident that we are using various legal authorities to look at metadata,” he said.
Mr Hawley, who was a leading figure in trying to block Mr Biden’s win and who was pictured giving a clenched fist salute to Mr Trump’s supporters, pushed back against Mr Wray.
“How are we going to know what you’re doing with it and how are we going to evaluate the bureau’s conduct if we don’t know what authorities you’re invoking, what precisely you’re doing, what you’re retaining?” said Mr Hawley. “You’re basically saying ‘just trust us.’”