A Black Lives Matter activist was part of the group that entered the U.S. Capitol on Wednesday.
Photographs showed Sullivan inside the Capitol in Washington on Jan. 6. Sullivan has since given interviews claiming he took part in the illegal breach of the building as part of an effort to understand supporters of President Donald Trump.
Sullivan has not been charged with unlawful entry or any other charges that police say other people who entered the building face. His picture is not among those circulated by authorities of persons of interest in the incident. Sullivan said he was detained on Thursday night and questioned about what he saw during the storming of the Capitol. The Metropolitan Police Department didn't respond to a request for comment.
Police said Thursday that 68 people had been arrested in the city, including 41 on Capitol grounds. Charges included unlawful entry and unlawful possession of a firearm, according to arrest data.
Sullivan recounted being near the woman who was shot dead by a U.S. Capitol Police officer.
"There was a glass wall, and she, the woman, was the first person to actually try to get inside," Sullivan told KSL. "All you see is hands come out the doorways with their guns. ... You don't see their face, nothing. And I literally yell at everybody else, 'There's a gun! There's a gun! Don't go in there!' And a shot goes off. And she gets shot as soon as she goes through."
The officer was placed on administrative leave pending a joint investigation conducted by the agency and the Metropolitan Police Department.
Other people who stormed the Capitol have been identified as supporters of President Donald Trump. That includes Derrick Evans, a member of the West Virginia House of Delegates, and Kristina Malimon, part of the Young Republicans of Oregon's leadership.