Some reports are speculating that there was actually two planes, the collided one being sent to the same runway as the first in error. Hence the pilot/co-pilot of the Black Hawk may have been referring to the first, when they gave the usual code, 'I see it' (or whatever it is), not realising there was a second plane right behind they had to 'go behind'. (This terminology is confusing: does it mean wait a tick or literally behind the plane once it's passed'?)
Anyway seems to be an ATC error, juggling too many balls at a time, spreading themself thinly. I expect all the ethnic minorities in the USA are praying that the ATC concerned isn't one of them because then Trump will feel vindicated and probably need to be carted off in a strait jacket.
Oh America, what have you done?