They marched into the thriving black Tulsa neighborhood on May 31, 1921, with shotguns and Molotov cocktails. Some wore masks, while others proudly showed their white faces.

Over the next two days, the mob killed hundreds of African-Americans, drove thousands more from homes and looted businesses before burning them to the ground.

The rampage, known as the Tulsa Race Massacre of 1921, is among the worst incidents of racial violence in American history — and for nearly a century Oklahoma leaders seemed determined to ignore it.

Now a reckoning is underway. This week, Tulsa officials announced plans to excavate a plot of land that they believe is a mass grave containing many of the victims. The digging is scheduled to begin in April.

“We are committed to exploring what happened in 1921 through a collective and transparent process,” said Tulsa Mayor G.T. Bynum. “Filling gaps in our city’s history, and providing healing and justice to our community.”

Racial terrorism was common in the segregated South, and Oklahoma was no exception. The Equal Justice Initiative, a nonprofit based in Alabama that focuses on criminal justice reform, has documented 36 lynchings in Tulsa County between 1877 and 1950 — more than in any other county in the state.

Greenwood was something of a rarity in its heyday, a neighborhood founded after the Civil War that grew into a flourishing destination for blacks fleeing the Deep South just as an oil boom was getting underway. With its burgeoning black-owned businesses, it became nationally known as “Black Wall Street.”

But the success of Greenwood bred resentment in white Tulsa.

The violence there began like so many other incidents of that era — with rumors that a black man had tried to sexually assault a white woman. A 2001 report from a commission that studied the massacre concluded that most likely the man had tripped and bumped into the woman, who screamed and ran off.

As the news swirled, an armed mob assembled and stormed the neighborhood. For 18 hours, whites ran through the streets, shooting black residents and torching homes, churches, a hospital and a library.

Oklahoma officials initially put the death toll at 36, where it remained for decades. Only later did historians who studied the massacre arrive at the current estimate of 300.

Nonetheless, the massacre remained a footnote in Oklahoma history. Many Americans only learned about it last fall with the release of HBO’s superhero drama “Watchmen,” which focuses on race relations in Tulsa and includes a scene of the massacre.