The area in the Capital building where where the woman was shot by a man in a black suit and mask, with white shirt cuffs and silver gun, is the door to the Speaker's Lobby. I found a tour video that clearly shows the area. The shooter came from the first door on the left as you enter through the doors (where the shooting occurred) to the Speaker's Lobby.

https://history.house.gov/Education/Capitol-Tour/Behind-the-Scenes-Tour/