Treason in time of war would be a good start.
I’m not sure why they didn’t go for Treason. The only thing I can guess is Casey got some political pressure to pursue a criminal charge of murder. But now the politicians want it both ways (as they often do) they don’t want to admit there are enemy inside the wire because that shows they failed to protect us, but they want to give Purple hearts to the victims which means the enemy was inside the wire. Obviously they don’t realize that Congress doesn’t award Purple Hearts....its the only one that the enemy decides who gets it.