Mexico's bank regulator said on Wednesday it fined Citigroup's Mexican unit Banamex nearly 30 million pesos (U.S. $2.2 million) over lapses in oversight of loans to oil services company Oceanografia that cost the lender more than $500 million. A February investigation showed the bank had failed in its prudential responsibilities and lacked adequate internal controls, Mexico's National Banking and Securities Commission said in a statement. Banamex said in a statement it had paid the fine and would follow recommendations made by regulators. Banamex said its own investigations showed that the fraud was an "exceptional" case. Citigroup said in February it had uncovered at least $400 million in fraudulent loans that Banamex made to Mexican oil services company Oceanografia. That figure has since increased to more than $500 million.