Charles Sweenay and James Young, witnesses for the prosecution in the trial of the conspirators accused of assassinating Abraham Lincoln, had been prisoners of war at various times and locations, including the notorious Andersonville Prison in Georgia. Read their accounts starting on page 76 of the Lincoln Assassination Papers - Proceedings of the Court-Martial. While neither was asked any questions regarding their knowledge of the assassination, the prosecutor was, no doubt, making the court aware of the horrendous conditions imposed by the rebels (the defendants were Southerners) on captured Union soldiers.
Other accounts are available within the Lincoln Assassination Papers, too. Just search on terms such as "prison" or "rebel" within the publication.