Swindles in which someone overseas seeks access to a person’s bank account are so well known that most potential victims can spot them in seconds.

But one man found success by tweaking the formula, prosecutors say: Rather than trying to dupe an account holder into giving up information, he duped the bank. And instead of swindling a person, he tried to rob a country — of $27 million.

To carry out the elaborate scheme, prosecutors in New York said on Friday, the man, identified as Paul Gabriel Amos, 37, a Nigerian citizen who lived in Singapore, worked with others to create official-looking documents that instructed Citibank to wire the money in two dozen transactions to accounts that Mr. Amos and the others controlled around the world.

The money came from a Citibank account in New York held by the National Bank of Ethiopia, that country’s central bank. Prosecutors said the conspirators, contacted by Citibank to verify the transactions, posed as Ethiopian bank officials and approved the transfers.

The fraud was uncovered after several banks where the conspirators held accounts returned money to Citibank, saying they had been unable to process the transactions, and an official of the National Bank of Ethiopia said that it did not recognize the transactions, according to a complaint signed by an F.B.I. agent, Bryan Trebelhorn.

In October, Citibank received two dozen faxed requests for money to be wired, and it transferred $27 million to accounts controlled by the conspirators in Japan, South Korea, Australia, China, Cyprus and the United States, the complaint says.