TULSA, Okla. (AP) — The search for remains of victims of the 1921 Tulsa Race Massacre continued for a fourth day on Thursday with no remains found during an excavation of a cemetery.

“We continue to be optimistic,” that remains of victims will be found in Tulsa’s Oaklawn Cemetery, said state Archaeologist Kary Stackelbeck.

“We have only tested a portion of this area overall, so we have more to investigate and more to look for. I continue to be optimistic that we are in the right place,” Stackelbeck said.

The excavation that began Monday has been expanded. Searchers have added an augur probe to drill further into the soil and collect samples that could indicate the presence of human remains that could include clothing, caskets or personal items buried with a body.

On May 31 and June 1 in 1921, white residents looted and burned Tulsa’s black Greenwood District, killing as many as 300 people.

Items discovered thus far include broken bottles, animal bones, unidentified pieces of metal and a shell casing that is not believed to be related to the massacre.