Citing anonymous government officials, the Journal reported that the hackers were connected to a Russian cyber gang. Two other computer systems, at least one of connected to a U.S. government agency, were also attacked.
Citigroup denied the report. "We had no breach of the system and there were no losses, no customer losses, no bank losses," said Joe Petro, managing director of Citigroup's Security and Investigative services. "Any allegation that the FBI is working a case at Citigroup involving tens of millions of losses is just not true."
The Journal reported that the attack on Citigroup's Citibank subsidiary was detected over the summer, although it may have occurred up to one year earlier. The FBI, the National Security Agency, the Homeland Security Department and Citigroup worked together to investigate the attack.
Cyber crime is of increasing concern to businesses and the federal government, with President Barack Obama calling it one of the "most serious economic and national security challenges we face as a nation."
Obama is expected on Tuesday to announce the appointment of Howard A. Schmidt, a former eBay and Microsoft executive, as the government's cyber security coordinator.