In the video below. Edifying Others documented the suspicious moving of arms while there was an active shooter.

You can also see the video clip in this video that correlates with the account of Stacy Lippel seeing the Police shooter in the stairwell and coming down the hall. After they were done they exited the building. That is the video clip in the corner of the video of them moving arms she could not recognize. They hid them in the tote bag so that no one could ID them on the way out.