I'd say it's more likely that none of those agents knew anything and that they did exactly what they were trained to do. There's simply no reason to inform the agents who don't need to know of a plan to allow the assassination of their own protectee. If they have no active role in such a plan, they don't need to do anything other than their normal job. It makes it more believable anyway when they're scrambling around wondering how such a thing could happen.

All that would need to be done is for some higher ups to botch the security plan. Every agent simply does their job, maybe you involve the sniper team, but ultimately it's the lack of agents on that rooftop that allows the plan to work. And if the shooter knew the plan, it allows him to work around the agents that are there.