The Federal Bureau of Investigation is probing a computer-security breach targeting Citigroup Inc. that resulted in a theft of tens of millions of dollars by computer hackers who appear linked to a Russian cyber gang, according to government officials.

The attack took aim at Citigroup's Citibank subsidiary, which includes its North American retail bank and other businesses. It couldn't be learned whether the thieves gained access to Citibank's systems directly or through third parties.

The...