Thank you for your replies. I will have to look at his CMSR, but it indicates that he worked in Alexandria for the first part of his service. Since his regiment was active in engagements until the end of the war, and was in fact the last regiment to be mustered out after the war (they fought a lot of rear-guard actions) I assumed that those were conditions of parole - not active combat service. His CMSR indicates that his name appears on the Union prisoner rolls at Libby, and that his name is crossed off. Those records may have been lost, then, but a note was made contemporaneously.

The prison that I think he worked at later became a prison for Union deserters under Pettit (spelling?) who was courtmartialed for torturing prisoners - the Prince Street Jail. I will have to go back in my research, but I did find a reference to a Camp Banks on the Potomac at Alexandria.

On this page, a Camp Banks is mentioned in Service Records on this page http://www.geocities.com/Pentagon/Barracks/3627/companyd.html as well as on this page: http://www.barlowgenealogy.com/Military/unionjohn.html

So I know the camp existed, I am just not sure what its purpose was or exactly where it was located. I think it is the Prince Street jail, but can't be certain.

Are there any good sources for camps with locations?