I lived in a townhouse in Alexandria directly opposite a marina just south of the DCA airfield - I could see the river from my bedroom window. There's a unit of Coast Guard helicopters stationed there and they fly up and down the river.
Sometimes you would see Army blackhawks but most commonly it was Coast Guard - and they fly a version of the blackhawk there. My suspicion is that it was a Coast Guard helicopter not where it was supposed to be. The collision happened in the final approach to runway 33 - which is the slanted one that takes the smaller jets like the CRJ-700.