Apols for the crude graphic. But this shows the relative location of Route 5 (along I-395), the Pentagon, the DCA tower, and the Pentagon city cluster of high-rises including the new Amazon HQ2. At the altitude the helo was flying, it would be largely masked from the tower. explaining why they "didn't have a good fix".
It would also be good to know who was on board that helicopter and why it was Pete Hairgelseth.