The Federal Bureau of Investigation is investigating a computer security breach targeting Citibank in which a gang of Russian hackers stole tens of millions of dollars, the Wall Street Journal reported.

It couldn’t be learned if the alleged thieves, known as the Russian Business Network, attacked Citibank’s systems directly or through third parties, according the report. Citibank is a subsidiary of Citigroup Inc.

“It remains under investigation, and whether any of the money has been recovered couldn't be learned,” the Journal reported.

The same group also assaulted two other entities, including at least one U.S. government agency, people familiar with the matter told the Journal.

The attack had been detected over the summer, but investigators say it could have taken place months or even a year earlier.

Citigroup’s managing director of Security and Investigative services Joe Petro said there had no breach of the system. He also said allegations that the FBI was working on a case involving tens of millions of dollars in losses “is just not true.”