Also, there’s the angle, how did the assassin know that the most obvious sniper perch was unguarded?
Evidently there was a ladder in an alcove. Either he or an accomplice put it there beforehand. If the ground crew had been thorough, they would have seen that ladder and removed it. Either that, or posted someone there to see who used it.
There appear to be people in all of these agencies and the military who are loyal to The Revolution above any oaths they have taken to the USA.