By a member of the Afghan military, using a American rifle.
This story is not being widely circulated – to read it online you have to subscribe to the magazine.
The problem can be summarized briefly – when you have a foreign military, financed and equipped by Americans, working closely with American troops – you are bound to have some random killings of the Americans. There is no way this can be prevented.
In this case, it was an American general – who like most generals, never intended to leave the safety of the Pentagon.