U.S. authorities are investigating the theft of an estimated tens of millions of dollars from Citibank by hackers partly using Russian software tailored for the attack, according to a news report.

The security breach at the major U.S. bank was detected mid-year based on traffic from Internet addresses formerly used by the Russian Business Network gang, The Wall Street Journal said Tuesday, citing unnamed government sources. The Russian Business Network is a well-known group linked to malicious software, hacking, child pornography and spam. The Federal Bureau of Investigation is probing the case, the report said.

It was not known whether the money had been recovered and a Citibank representative said the company had not had any system breach or losses, according to the report.

The report left unclear who the money was stolen from but said a program called Black Energy, designed by a Russian hacker, was one tool used in the attack. The tool can be used to command a botnet, or a large group of computers infected by malware and controlled by an attacker, in assaults meant to take down target Web sites. This year a modified version of the software appeared online that could steal banking information, and in the Citi attack a version tailored to target the bank was used, the Journal said.