Who says they have to be DC critters. They should have all been outsiders like himself from day one. Just retain a few DC swamp-things as consultants and keep them at arm's length and out of the West Wing.
The problem with ‘outsiders’ , they are clueless where the moles hide. I do not think there was any other way, except to follow the raised dirt piles as these moles continued their dirty work.. set traps..and catch them in the act. Much of the dirt moving was/is done by contractors.