Since this sort of activity goes against most police department's code of conduct policy, the burden of proof that this actually happened is on you. I don't know of a single cop that will agree to randomly fight a citizen. Unless you previously have known him for a long time, I just don't see this happening. Care to divulge his name and department? Since you fought him, you should at least know who he works for, and his name - especially since it is on his name tag.