JP Morgan Chase has confirmed that it's working with law enforcement agencies in the US to determine the scale of a possible cyber attack.
A spokeswoman for the firm said it was taking steps to secure sensitive information but she added that the bank was not seeing any unusual fraud.
Earlier, the FBI said it was investigating media reports that several US financial firms had been the victim of cyber attacks.
Bloomberg News reported that Russian hackers were believed to have carried out cyber attacks against JPMorgan Chase and another unnamed US bank in mid-August, resulting in the loss of sensitive data.
Authorities are investigating whether the breaches were linked to recent attacks on major European banks, the report said.
The New York Times reported that the networks of JP Morgan and at least four other US banks had been infiltrated in a string of coordinated attacks this month, citing four people familiar with the investigation.
The attackers stole large quantities of data, including checking and savings account information, though their motivation is not yet clear, according to the Times report.
It added that several private security firms have been hired to conduct forensic reviews of infected networks.