The Official Records do not seem to have fixed characteristics for the two; there are instances of officers listed as "killed" who died anywhere from one to three hours later, and officers "mortally wounded" who died the same day. Should "killed" refer to someone who was killed instantly or semi-instantly, and "mortally wounded" for those thereafter?
It seems to me that there would need to be a time limit, morbid as it is, to fit the correct description. Should "killed" solely refer to those who died within 1 hour of their wounding? What would you qualify as "Killed" vs "Mortally wounded"?
The distinction of killed in action and mortally wounded could be a very fine line apparently. And officers had to make value judgements on reporting men "killed in action." It seems generally to have meant men who were not given significant medical treatment on the field before their death during or about the time of action. As in the case of those certainly
found dead on or about the field of action.
Coroners were of course not on hand to examine the bodies to determine
time of death. It appears those presumed "killed in action" were certainly those deprived of life, etc. "in action" in front of the enemy, etc.
Even those "killed" usually died after a short period, however long. Some were seemingly dead before they hit the ground...
Lt. Col. Chandler of the 88th Illinois died almost instantly when killed in action at Kennesaw Mountain.
Some perhaps not so instantly, as with Henry Ropes at Gettysburg...
Colonel Roberts of the 1st Michigan at Second Manassas died in about ten minutes after informing his men he was
killed...
Both the killed and wounded were of course wounded, and in action. Besides those classed as "killed" the men officially "wounded in action," were alive or showed signs of life when recovered or carried off the field apparently. Whether the wounds were proved fatal or not, was a question of time. Some died before they could even be carried to the rear or given aid. They appear to have been a large proportion of those later recorded as "killed in action" generally. Of the wounded, the
mortally wounded were those among them who died within hours, days, weeks or even months later while under medical treatment etc.
From the 45th Massachusetts's Volunteer Militia history, we have this notice on the difficulties of officers in assigning the classifications. The Governor of Massachusetts's argues the 45th suffered 10 "killed in action" on the rolls and accuses the regimental commander of lying when he reported 21 "killed in action." Edward Winslow Hincks notes here how silly the arguments could become given the realities of the battlefields: