LAGOS (Reuters) - The chief executive of Nigeria's Fidelity Bank remained in custody on Tuesday, having been detained last week by the country's financial crimes agency, prompting the lender to appoint a temporary replacement. A spokesman for Nigeria's Economic and Financial Crimes Commission (EFCC) said the agency obtained a court order to detain Fidelity Nnamdi Okonkwo and that he would remain in custody while the investigation continued. "It is an ongoing investigation that requires his attention and that was why he was arrested. He has been assisting the commission with the investigation," Wilson Uwujaren said. Okonkwo has been in the custody of the EFCC since Wednesday, as part of investigations into transactions made in the run-up to presidential elections last year. The bank has said that "the transactions were duly reported as required by the regulators" and that it is cooperating with the authorities. On Monday the bank appointed one of its executive directors Mohammed Lawal Balarabe as acting chief executive with "immediate effect". Shares in the bank fell nearly 8 percent to a two-month low of 1.08 naira on Tuesday, the first day of trading since it reported a 14 percent drop in first-quarter profits after the market's close on Friday. The shares are down 22 percent so far this year. President Muhammadu Buhari took office last May after winning an election on a campaign promise to crack down on corruption, and the EFCC has made a number of high-profile arrests since. (Reporting by Chijioke Ohuocha, Oludare Mayowa and Alexis Akwagyiram; Editing by Louise Heavens, Greg Mahlich)