The FBI is investigating a hack at Citigroup which is thought to have been carried out by a gang from Russia.
According to the Wall Street Journal, which quoted government officials, the attack was on the group's Citibank subsidiary.
The report said the attack was detected over the summer, but investigators are looking into the possibility that the attack may have occurred months or even a year earlier.
It also said FBI, National Security Agency and Department of Homeland Security and Citigroup are all involved in the investigations.
Citigroup's Security and Investigative services managing director, Joe Petro, told the Wall Street Journal that there had been no breach of the system and no customer losses or bank losses. "Any allegation that the FBI is working a case at Citigroup involving tens of millions of losses is just not true," he said.