Almost $750 million paid by international mining companies to the Democratic Republic of Congo’s state-owned miner over a three-year period are missing from the company’s accounts, the Carter Center said.

Royalties, signing bonuses and asset-sale proceeds due to Gecamines from more than 20 copper deals with partners between 2011 and 2014 can’t be reliably tracked to Gecamines’ accounts, the Atlanta-based advocacy group said in a report published Friday. That’s almost two-thirds of the $1.1 billion in partnership revenue Gecamines should have received, according to the analysis based on a review of contracts, corporate records, public statements and more than 200 interviews.

Some of the money missing from the company’s accounts was traced to “specific destinations, including debt repayments, asset acquisitions, and modest infrastructure investment,” while “the bulk of the missing revenue could not be traced.”

Gecamines President Albert Yuma and Director-General Jacques Kamenga didn’t respond to calls seeking comment. Valery Mukasa, chief of staff to Congo’s mines minister, said he had yet to see the report and declined to comment.

Congo is Africa’s biggest copper producer and the world’s largest source of cobalt. A World Bank-supported mining law in 2002 attracted billions of dollars of investment into copper and cobalt projects from mining companies including Glencore Plc, Freeport-McMoRan Inc. and China Molybdenum Co. While the economy has grown, the economic benefits of increased copper production haven’t been transferred to the population, two-thirds of whom live on less than $2 a day. Read the full report here.