This image from 1920 shows the the north facade of Smithsonian Castle. (Courtesy of Smithsonian Institution Archives)

The iconic red sandstone used to build the Smithsonian Castle, one of Washington’s most recognizable buildings, was quarried by slaves, including some who were once most likely owned by Martha Washington, according to new historical research to be published Thursday.

The discovery by anthropology professor Mark Auslander adds to what has been a years-long reckoning with slavery’s role in Washington landmarks, including the Capitol and the White House, and adds nuance to the historical portrait of the Smithsonian Castle, which was built between 1847 and 1855.

Auslander, a native Washingtonian, said in an interview that the Smithsonian has been reluctant over the years to address whether slave labor might have played a part in the history of the Castle. “It’s just an area of total silence,” said Auslander, whose findings are to be unveiled in Southern Spaces, an online, peer-reviewed journal published in cooperation with Emory University’s Robert W. Woodruff Library. “The Smithsonian hasn’t gone through the truth-and-reconciliation process that a lot of institutions have gone through. But I think there’s a willingness to do so.”

Smithsonian officials did not respond to repeated requests for comment.

Auslander, who teaches at Central Washington University, does not contend that slaves participated directly in the building of the Castle, beyond the hard, perilous work of quarrying the stone. In this way, the Smithsonian appears to differ from the White House and the Capitol, which were partially built by slaves.

He says that references to unnamed “colored men” on a Smithsonian landscaping work order suggest that slaves could have labored at the construction site. If they had been free African American men, their names probably would have been listed, said Auslander, the author of the book “The Accidental Slaveowner: Revisiting a Myth of the American South.”

Auslander plumbed old Smithsonian ledger books, with the permission of Smithsonian officials, while he was a senior fellow at the National Museum of African Art last year. The sandstone, he learned, was bought from a quarry in the community of Seneca in Montgomery County. That quarry was owned by John Parke Custis Peter, a great-grandson of Martha Washington’s who had inherited slaves she once owned. (George Washington had famously freed his slaves in his will, but his wife retained slaves she had owned during a previous marriage.)

An inventory of Peter’s possessions after his death in 1848 includes slaves named Sandy, George, Davy and John, all of whom are listed as being older than 50, as well as an “old” woman named Celia. The names matched the names of infants and young children in records of Washington’s possessions at Mount Vernon before her death in 1802.

“I was shocked as I followed the bread crumbs that nobody had looked into this,” Auslander said.

The crumbs led Auslander to Seneca, where he says he interviewed African Americans who live near the old quarry site. They said that stories about slaves quarrying the stone for the Smithsonian Castle were a part of local lore but that they never heard any official confirmation.

When he told them he was researching the topic, some residents exclaimed, “We’ve been waiting for you!” Auslander said. “For them it had just been oral history.”

It isn’t anymore.

Megan McDonough contributed to this report.