The total strength of government building security in DC is around 2 battaglions.
You think there is nobody who can bring 2000-3000 people to the rifle, and numerically overwhelm them? And it seems that not insubstantial numbers of DC security personnel did simply desert their posts.
Commando paratroopers with heavy weapons, and moreover armour will leave no chances for a regular army unit, let alone civilian police.