The bank said it is cooperating fully with the EFCC on the investigation.
During the build-up to the 2015 presidential election, Diezani reportedly invited Okonkwo to help her handle some cash, which was meant to be disbursed to electoral officials and groups.
Upon investigations, the Economic Financial Crimes Commission (EFCC), allegedly discovered that some INEC officials in Rivers, Delta and Akwa Ibom states received N675.1 million from the fund.
In a statement issued by the management of the bank on Thursday, April 28, it said the "transactions were duly reported as required by the regulators and the Bank is cooperating fully with the authorities on the investigation.
"We assure our numerous stakeholders, including our customers that we are working assiduously towards a quick resolution of the issues."
Okonkwo was reportedly arrested alongside some other officials of the bank, including its Head of Operations, Mr. Martins Izuogbe, over the alleged fraud.