DCA has two runways. The longer runway is aligned with the river’s north/south direction at that point, so planes flying the approach to this runway from the south track along the western shore of the river, leaving the eastern shore clear for other aircraft like helicopters. The vast majority of flights on approach from the south [1] use this longer runway.
The secondary runway is set at a diagonal NW/SE. Planes flying an approach from the south follow the river at first, but then loop out over the eastern shore of the river to line up on that runway. To my eye the radar track of the downed flight follows this path. It’s possible since it was a small plane and only small planes can use the diagonal runway—it’s shorter.
I mention this because this track takes planes into airspace that is a) usually clear of commercial airplane traffic, and b) directly over military facilities like Naval Research Lab and Joint Base Bolling, which have significant military helicopter travel.
Basically, I wonder to what extent the helicopter pilot was surprised to find an airplane descending in that location.
[1] When flights are approaching from the north, the main runway requires a pretty sharp right turn seconds before touching down. Approaches to the diagonal runway from the north take planes almost directly over the Pentagon.