WASHINGTON (AP) — The National Park Service has acquired a rare Civil War-era photograph of an enslaved woman who helped save Confederate Gen. Robert E. Lee's home in Arlington, Virginia.

The previously unknown photograph depicts Selina Gray, the head housekeeper to Lee and his family, along with two girls. The photograph was unveiled Thursday at the Arlington House plantation overlooking the nation's capital that was home to Lee and dozens of slaves before the Civil War.

An inscription on the back of the image reads "Gen Lees Slaves Arlington Va."

Park officials said this is only the second known photograph taken of slaves at Arlington.

"It's extremely rare to have an identified photo of an enslaved person," said National Park Service spokeswoman Jenny Anzelmo-Sarles. "Since slaves were considered property, it's very rare to have a photo where you can identify the people in the photo."

Gray is noted in history books for helping to save Arlington House after Lee's family left and the plantation was captured by Union troops during the Civil War.

Arlington House was originally built as a monument to George Washington. Lee's wife, Mary Custis Lee, entrusted the home to Gray, and she later confronted a Union general about soldiers pilfering Washington family heirlooms from the house. She was able to have the items safeguarded.

The photograph was purchased on eBay in September for $700 after a volunteer found it online. The seller was based in England and found the photo in a box of unwanted images. The nonprofit Save Arlington House Inc. donated funds to acquire the image.

The photograph will be unveiled to the public Saturday, and it will be used in future exhibits after Arlington House and its slave quarters are restored over the next two years. Historians will study the image and hope to learn more about it.