U.S. Treasury Secretary Steven Mnuchin has rejected plans by the African Development Bank’s board to end an investigation into its president, Akinwumi Adesina, and called for an independent probe into allegations against him

In a letter dated May 22 and addressed to Niale Kaba, chairwoman of the bank’s board of governors, Mnuchin said the Treasury disagrees with findings by the bank’s ethics committee that “totally exonerated” Adesina.

The intervention by the Treasury, the AfDB’s biggest non-African shareholder, comes two weeks after the ethics committee found no evidence to support allegations of favoritism by Adesina.

Considering the scope, seriousness, and detail of these allegations against the sole candidate for bank leadership over the next five years, we believe that further inquiry is necessary to ensure that the AfDB’s president has broad support, confidence, and a clear mandate from shareholders.

“Considering the scope, seriousness, and detail of these allegations against the sole candidate for bank leadership over the next five years, we believe that further inquiry is necessary to ensure that the AfDB’s president has broad support, confidence, and a clear mandate from shareholders,” Mnuchin said.

U.S. criticism of the bank’s internal processes follows comments by World Bank President David Malpass in February that multilateral lenders including the AfDB tend to provide loans too quickly, and, in the process, add to African nations’ debt problems.