A prison in Washington, D.C. for Confederate soldiers as well as for political prisoners and important people suspected of being disloyal. The latter two groups were often denied the writ of habeaus corpus. The granite prison building had once been the nation's temporary Capitol after the British had burned the Capitol during the War of 1812. It was then a hotel before becoming a prison in July 1861.
It had an annex known as Carroll Prison. Famous Civil War prisoners in the Old Capitol included the flamboyant raider, Major John Singleton Mosby, know as the Gray Ghost, and the Confederate spy, Mrs. Rose O'Neal Greenhow.
The Old Capitol also held prisoners from the battle of Gettysburg. One recorded happy ending involved the Confederate spy, Miss Antonia Ford of Fairfax, Virginia, who wed her guard (Captain Willard of a distinguished hotel family) when she was released.
After the war, the only person ever executed as a war criminal, Major Henry Wirz, the commander of the infamous Confederate prison at Andersonville, was hanged at the prison on November 10, 1865, as soldiers chanted nonstop "Andersonville, Andersonville, Andersonville".
The site of the Old Capitol Prison is today occupied by the U.S. Supreme Court Building.
From: "The Language of the Civil War John D. Wright, page 211.