WASHINGTON (AP) — New research has found that the red sandstone for one of the oldest buildings on the National Mall, the Smithsonian Castle, was quarried by slaves.

The Washington Post  reports that anthropology professor Mark Auslander of Central Washington University is publishing new work that shows some slaves likely owned by Martha Washington were responsible for quarrying the stone.

Auslander does not contend that slaves participated directly in building the Castle. That differs from the Capitol and White House, which were partially built by slaves.

The Smithsonian building is a Washington landmark. It was built between 1847 and 1855.

Auslander’s findings are being published Thursday with Emory University in Southern Spaces, a peer-reviewed journal. His studies revealed the sandstone came from a Maryland quarry owned by a great-grandson of Washington.

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