This occurred on a military base, so the Afghans are out of the investigative picture, I'm told. Let the NTSB review the evidence and announce the cause.
Well they are out of the picture, all right. But that is just because they happen to be a country occupied by american forces and have no say in anything, so...
Other than that, the international law is pretty clear here:
The accident occured within Afghan airspace/territory.
Furthermore this was a civil aircraft and not a military/state aircraft.
The investigation is therefore under the authority of the Afghans - read Art. 26 Chicago Convention.
In the unlikely case that an Afghan investigator should show up at the gates of Bagram airbase, they would have to let him in to do his job.