The gun had already been tossed behind the fence. I know this because the image you posted was after the cop fired and the suspect was on his way down. If he had a gun in his hand as you implied, the gun would have been very close to the body. The gun was found about 10' behind the fence opening, clearly indicating the suspect had already tossed the gun just prior to being shot.
Hopefully the fingerprints on the gun will help.