AILSA CHANG, HOST:

Washington, D.C., is still tense today as Army troops erected a new defensive barrier around the Capitol building. And Mayor Muriel Bowser has extended an emergency declaration for her city through Inauguration Day. And there are new developments tonight. The chief of the Capitol Police announced that he is stepping down, and Senate Majority Leader Mitch McConnell has ousted the Senate sergeant-at-arms. This comes as law enforcement scrambles to find and arrest those involved in yesterday's violence. NPR's Brian Mann joins us now with more.

Hey, Brian.

BRIAN MANN, BYLINE: Hey, Ailsa.

CHANG: All right. So let's start with these resignations. We've got Capitol Police Chief Steven Sund and Senate Sergeant-at-Arms Michael Stenger. Can you just tell us more about their departures here?

MANN: Yeah. So we're seeing the start of a major shake-up here. It was Steven Sund's job to protect Congress and the buildings where lawmakers and staff work. His resignation letter offered no new information about what went wrong yesterday. Sund just said he's going to go on sick leave and will then leave his post permanently. And as you mentioned, the Senate majority leader, Mitch McConnell, also forced the immediate resignation today of Sergeant-at-Arms Michael Stenger. He's also charged with protecting senators. All of these moves followed D.C. Mayor Muriel Bowser calling today for a bipartisan congressional probe of what went wrong on their watch. She called this a catastrophic security failure and said it can't be allowed to happen again.

CHANG: Well, let's move on now to the arrests because, I mean, one question that's being asked a lot right now is why there have been so few arrests. You know, this was a huge crowd. People were caught on video vandalizing the Capitol, in some cases assaulting police officers. Why aren't more people in custody right now?

MANN: Yeah. And again, some of this comes back to Steven Sund at the Capitol Police. I talked today with one of the senior federal law enforcement officials investigating the riot. NPR agreed not to use his name so he could give candid views about the law enforcement response. And he sounded really angry. He told NPR it appears Capitol Police had the opportunity to arrest some of the rioters and did not. We've asked Capitol Police about this. They have not responded to our questions. And so now what we're seeing is law enforcement playing catch-up. They're using social media and other video recordings that are out there to try to catch those who committed crimes. Chief Robert Contee, who heads the local Metro D.C. police, talked about this effort today.

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ROBERT CONTEE: We still have a significant amount of work ahead of us to identify and hold each and every one of the violent mob accountable for their actions. We have collected numerous images of persons of interest that we are asking the community to help us identify.

MANN: And Contee said they're also sweeping local hotels and other places around the city, trying to identify any suspects, Ailsa, who might still be in the Washington, D.C., area.

CHANG: Well, there was also a Justice Department briefing this afternoon. What do federal officials say about their ongoing response to everything that happened yesterday?

MANN: Yeah, what they're describing is a full-court press with all the federal law enforcement agencies. Michael Sherwin, who's the acting U.S. attorney for the District of Columbia, said more than 50 federal cases are active right now, including eight involving pretty serious firearm charges - said one man was arrested with what he described as a military semiautomatic assault rifle and Molotov cocktails. And of course, they're also investigating the placement of pipe bombs outside the RNC and DNC headquarters.

CHANG: And real quick, do we know at this point how well organized all of these riots were, whether they involved militia groups or any other groups?

MANN: We really don't know yet. Sherwin said they're looking to see if there was any kind of command and control here. He says if there is some investigation that finds orchestration, there could be conspiracy and insurrection charges.

CHANG: All right. That is NPR's Brian Mann.

Thank you, Brian.

MANN: Thank you so much. Transcript provided by NPR, Copyright NPR.