Comments:

William F Mitchell was a soldier who was convicted by court marshal of some military offense and sentenced to the prison on Alcatraz, where he died.  The lack of a cross or other emblem on his tombstone reflects his inmate status.  Army inmates on Alcatraz were designated either as "general prisoners" or "military convicts."  General prisoners were sentenced to Alcatraz for a period of time, but after that had been served they would be returned to their units and allowed to serve the rest of their enlistment.   Military convicts like Mitchell, however, had been dishonorably discharged and were spending the rest of their enlistment periods (or longer, if the offense was serious enough) in confinement. 

As of April 2004 the VA grave locator also listed Mitchell as a civilian on Wake Island Naval Air Station at the time of his death, obviously an error.  The VA database notes that he was originally interred in grave 118 on Angel Island.