Sounding like this will fall more on the ATC, but also a bit on the Helo crew. ATC allowed them to be in that airspace and deconflict visually. ATC did not have to approve that flight pattern. ATC wasn't specific on where the deconfliction needed to take place, so that may have caused confusion. Helo crew thought they were clear and didn't keep their head on a swivel while in condensed airspace. Ultimately both should have had more concise communication and visually verified what they were being told.
Spoke this morning with a co-worker who is a retired Navy helo pilot. He flew in that DC / Reagan space for years and years. With all the usual caveats that this is sad, human life was lost, etc my colleague was adamant that the helo was flying higher than it should for that route, that it was absolutely the helo pilot’s responsibility and mistake, unless ATC specifically told him/her to be at that altitude in that space.
This guy is all business, no-nonsense. He loves the military and everything about the helo world. He was pretty shaken up about the whole thing but also very clinical. Sounded like he spent 2-3 hours exploring this today.
(Aside: he works on my team….in my mind I was like, dude, don’t you have stuff to do instead? And then I realized how much I post on FBGs)