From the plaque:
On this site stood the Calhoun House, used as a Federal Courthouse, where desperado Frank James was tried and found not guilty, by jury trial, on April 25, 1884, for robbery of a government payroll near Muscle Shoals, Alabama. May 11, 1881.
One of his defense attorneys was Huntsville's LeRoy Pope Walker, first Secretary of War of the Confederate States of America.