At this point, any number of others might have been involved or at least help enable the crimes. Though, the two of them being brothers suggests it might well have just been them, and that the evidence stops there. Unknown what the sum total of info gotten out of the surviving guy has been, so far, whether the investigators have gotten any planning documents and related evidence that shows it stops with them, etc.
But, the security group charged with protection at the event complicit in the murders? Can't imagine the organization is. Might be possible that one or more other individuals assisted in various aspects. Who's to say, at this point? Right now, unless hard evidence comes up, it's all supposition and "conspiracy" theories.
