Not sure who did it, but in the photo above you can see a man with a suppressed AR-15 short barreled rifle. He appears to me to have an Aimpoint optic, perhaps an Aimpoint Pro. Also looks like a Magpul forward grip. The young woman was shot in the neck and last I heard, she was in “critical” condition. The man in the photo above was standing right over the woman after the gunshot went off in the video. He is clearly not a civilian.
The shooter can clearly been seen in the second video on the left hand side through the door.
If any of y’all have any other info or input, post in the comments. AP has people on the ground in DC. Pray for them, and pray for the movement.
EDIT: The woman was identified as Ashli Babbit, a 14 year veteran of the USAF and a huge Trump supporter. Her husband said that “she was a strong supporter of President Trump, and was a great patriot to all who knew her.”

According to Moustache6actual on Gab:
There is another video angle in which you can clearly see who shot her. It was an officer posted in the second doorway on the leftside of the hallway (on the opposite side of barricade). The red circle shows the purported shooter and the arrow points to the victim, Ashli Babbit, described above.