I agree. My hearing is slipping away so I had trouble sorting out the sounds amidst all the crowd noise and panic.
I originally suspected an air gun because it was assumed someone snuck it inside the perimeter protected by, presumably, magnetometers and other measures.
Now we know there was a serious gap in the perimeter.
A CIA guy on FOX said the perimeter should have been 1,000 yards; but, we now know there was ingress and blind spot for the shooter to reach 130 yards.
This is a fact that makes my suspicious brain light up, remembering similar intelligence-agency-type nuances such as the disappearance of 9/11 Flight 77 in an FCC transponder blind spot.