I don't think the shooters were identified, and the order was to get rid of the bodies. Even though Michael gave Rocco the order to keep them alive, you don't see him exactly broken up over it.

Few scenarios - there was an inside man with the shooting team to dispose of the other shooters, they shot themselves, or an overeager Corleone button shot them. I doubt it was Fredo. Deanna was hysterical after seeing the bodies, and you see Fredo dragging her away in his robe and pajamas. Besides, we know from the attempted hit on Vito that Fredo is an expert with firearms.

Which reminds me...just how did Fredo make his bones anyway?