This comment was written by a US Coast Guard helicopter pilot. It gives a lot of information on how the two aircraft should have been able to share the space and some speculation on what went wrong:
A helicopter instructor suggests that possibly the helicopter pilots, who were told to go behind an aircraft that was landing, were looking at the previous aircraft that was landing.[1] That's just speculation at this point.
Specifically the Baltimore-Washington route chart was relevant for these flights: https://aeronav.faa.gov/visual/10-31-2024/PDFs/Balt-Wash_Hel...
If you find the DCA airport on that chart, you’ll find routes 1 and 4 which roughly correspond to the helicopter’s flight path.