TULSA (AP) — A team of researchers and historians on Monday resumed test excavations of potential unmarked mass graves from the 1921 Tulsa Race Massacre.

A backhoe operator began slowly moving dirt at Tulsa’s Oaklawn Cemetery, where ground-penetrating radar earlier this year determined there was an anomaly consistent with mass graves.

Researchers plan to open a 6-by-3-meter excavation area using the backhoe to clear the first layer of soil, followed by shovels, trowels and even more delicate tools if remains are uncovered, said Oklahoma State Archaeologist Kary Stackelbeck. “It can become a tedious process, but it’s important that we do this carefully, cautiously and take copious notes along the way,” Stackelbeck said.

On May 31 and June 1 in 1921, white residents and civil society leaders looted and burned Tulsa’s Black Greenwood district, known as Black Wall Street, to the ground, and used planes to drop projectiles on it.

The attackers killed up to 300 black Tulsans, and forced survivors for a time to live in internment camps overseen by National Guard members.

In the years that followed, Tulsa city and business leaders engaged in a “concerted cover-up” to hide the truth about the massacre, Tulsa Mayor G.T. Bynum said.

“You had generations of people who grew up in this community ... and never heard about it,” Bynum said.