Investigations by the Defense Department or US and Afghan military officers is not good enough. While most of our military may be noble and well-intentioned, the best way to prove this would be with an independent, impartial investigation that adheres to procedures established by international law and the Geneva Conventions.
Especially considering:
-Doctors Without Borders provided its GPS coordinates to the military many times
-The bombing continued for 30 minutes after it was reported by the hospital
-US Afghan allies have resented this hospital for treating Taliban soldiers
-Afghan officials defended the attack by saying Taliban were inside
-The US has a history of attacking civilian targets in questionable circumstances (example here: http://tiny.cc/civiliantarget )