In order to take over such a system one would need to know the identifier signal.
Then replicate it and play it back in such a way that the UAV 'believes' you rather than its rightful controller. Not only which
my signals, which after all have only the same validity as the original owners, have to be more powerful and more 'believable'. Otherwise, all that will happen is a series of conflicting commands at which, I suspect, the onboard signal processors will simply throw up their electronic hands, give up the struggle and leave the aircraft in the hands of its onboard 'get home safe' processor and its protocols.
Did the wayward UAV not simply loose signal - for whatever reason - and wander off on its own, which happened to be toward Washington DC?
Roger.