That's an astoundingly easy thing to do with aircraft at night.
This is the real problem. The helicopters in question are based on the other side of the approach from their ordinary operations area (i.e., metropolitan DC). We have a similar problem at my international airport: the Air Force operations areas are across the northern approaches from the base. However, in the Reagan Airport case the distances and altitudes required to deconflict the traffic are much, much smaller.
As usual, Blancolirio offers very good commentary from an expert point of view, and avoids unnecessary speculation. What he doesn't touch on (but probably will when he can) is the capacity creep that has plagued both the military and civilian traffic. This is an inherently gnarly traffic-control problem, and under pressure from government officials there has been a steady relaxation of ordinary capacity constraints. While we will almost certainly identify errors on the part of various humans involved, this accident could be considered inevitable.