Hi Mike -
The apparent execution of African-American Union soldiers involving both Col. John Griffith and Col. Frank Powers has always been an "eye-opener" for me as a descendant of Col. Griffith.
Without having been there, the extant documentation does seem to indicate that some sort of summary execution (via shooting and/or hanging) did occur. Powers' statement to Col. Logan (in O.R. and in URL given below) seems to indicate that the captured Negro soldiers may have "made a break for it" and the shooting started, but other statements indicate that some of the captured Negroes were summarily hung.
I have always thought that Col. Griffith's involvement in this incident prevented him from higher command within the CSA. If this is true, his involvement may have been more than the extant documentation suggests.
Do you have some additional statements from the 9th Tenn. Cav. Battal. that you can share with us???