A haunting 150-year-old photo found in a North Carolina attic shows a young black child named John, barefoot and wearing ragged clothes, perched on a barrel next to another unidentified young boy.

Art historians believe it's an extremely rare Civil War-era photograph of children who were either slaves at the time or recently emancipated.

The photo, which may have been taken in the early 1860s, was a testament to a dark part of American history, said Will Stapp, a photographic historian and founding curator of the National Portrait Gallery's photographs department at the Smithsonian Institution.

In April, the photo was found at a moving sale in Charlotte, accompanied by a document detailing the sale of John for $1,150, not a small sum in 1854....