U.S. researchers have begun searching for the unmarked mass graves of an estimated 300 African Americans who were killed in the Tulsa Race Massacre nearly a century ago.Test excavations began Monday in Oaklawn Cemetery in Tulsa, Oklahoma, located in the south-central region of the U.S., after a radar search earlier this year revealed the possibility of mass graves.The search was postponed in March because of the coronavirus pandemic and involves the use of a backhoe to remove the first layer of soil before more delicate tools are used if remains are discovered, said Oklahoma State Archaeologist Kary Stackelbeck.The Tulsa Race Massacre, also known as the Tulsa Race Riot, during which white mobs murdered Black residents and destroyed businesses in an affluent area known as Black Wall Street, is arguably the worst outbreak of racial violence in U.S. history.Eighteen hours of violence that erupted on May 31, 1921, left the city’s prosperous Black community of Greenwood burned to the ground.  A stone memorial to the 1921Black Wall Street Massacre sits across from Vernon A.M.E Church in Tulsa, Oklahoma, U.S., June 18, 2020.Nearly 10,000 people were left homeless after the mobs, which included members deputized by Tulsa police, torched over 1,400 homes, scores of businesses, a dozen churches, a hospital, a school and a public library over 35 square blocks in the Greenwood community.The May 31 riot was sparked by a confrontation between a white lynch mob and Black men who were protecting Dick Rowland, an African American teenager who was accused the day before of trying to rape a white elevator operator. For nearly two days, white mobs torched the community, leaving it in ruins. Rowland was eventually exonerated.An all-white grand jury blamed the mayhem on Black city residents. No whites were ever imprisoned for the murders and acts of arson, even with overwhelming evidence.Most of Tulsa’s Black population were pushed into homelessness by the violence. Many began rebuilding Greenwood within days, despite the efforts of white power brokers who tried to force them to relocate.The massacre erupted during a period of especially heightened racial tensions in the United States. In the summer of 1919, race riots broke out and continued into the fall, spreading to at least 26 cities in some of the most intense racial violence in recorded U.S. history.Tulsa government and business leaders participated in a “concerted cover-up” of the Tulsa Race Massacre for years after it occurred, said Tulsa Mayor G.T. Bynum.“You had generations of people who grew up in this community … and never heard about it,” he said. “I feel a tremendous responsibility as mayor to try and find these folks. That’s a basic thing that a city government should do for people, and Tulsa hasn’t.”Tulsa officials said they expect the excavation to take up to six days.