The guy that shot her was wearing a suit without any clear badging. This looks like Secret Service who may have been on VP Pence detail.

[Viewer discretion advised]

Screenshot:

https://i.imgur.com/vHvvkHQ.png

Video:

https://worldstar.com/video.php?v=wshh17cIq2bB6L71wx1q

Secret Service protect high-profile Congressmen and Senators. For example: Former first-ladies, presidential candidates, and others as appropriate. Hillary Clinton, for example, had an entourage of secret service agents wherever she went, including in the Capitol. Same with Obama leading up to the election.

TLDR: It's likely there are secret service members in the Captiol at any given time.

It does seem to have been a plainclothes USCP officer per new reporting. I had read possibly erroneous reports that he was part of Pence's detail.