It seems that at around 3 seconds, Sterling hand moves, and at 4 seconds, his right arm comes back and he pulls his head off the ground. That is when one officer says he has a gun. Both officers then reach for their guns as well (the one on top of Sterling visibly does so at the 5-second mark, but I don't have a still photo of it here) and the officer on top of Sterling also apparently wrestles with Sterling's right arm for several seconds. At the 6-second mark, one can see the officer's hand apparently grabbing Sterling's, and several seconds later, just before the first shot, their hands again appear that way. For several seconds the officer on top and Sterling seem to wrestle along Sterling's right arm and side, and at around 9 seconds the camera jerks and the wrestling intensifies and at around 10 seconds is the first gunshot coinciding with the officer on top either flinging himself off Sterling or being thrown off of him as Sterling partly sits up. Once the camera is turned back on the officers and Sterling, it seems that the officer with his gun drawn is likely the officer who was on top of Sterling.
Sterling’s arm is partly underneath the car and I wonder if his gun was on that side in his belt or pocket.