Hackers believed to be from Russia broke into the computer systems of JPMorgan Chase and a second US bank earlier this month, sparking a federal investigation, US media reported Wednesday.
Bloomberg said two people familiar with the probe confirmed that the Federal Bureau of Investigation was examining the case to see if it is retaliation for US sanctions against Moscow over its support of Ukraine's secessionist rebels.
Bloomberg and The Wall Street Journal, which also reported the hacking case but without naming Russians as behind it, said it was not clear what damage the hackers caused or what data they may have stolen.
Bloomberg said the hackers showed a high level of skill to get through layers of security in the bank's systems, "a feat several security experts said appeared far beyond the capability of ordinary criminal hackers."
It said investigators, who also include the National Security Agency, are also studying whether the attack may have come from criminals in Russia or eastern Europe.
They are also examining whether the online break-in is related to similar incidents involving European banks.
The FBI declined to comment.
A JPMorgan spokeswoman did not confirm the specific case, but said in a statement that "Companies of our size unfortunately experience cyber attacks nearly every day."
"We have multiple layers of defense to counteract any threats and constantly monitor fraud levels."
A source familiar with the matter said the bank regularly informs law enforcement agencies when it sees suspicious activity.
"A critical part of our response is monitoring for fraud," the source said, but "we are not seeing any increased activity at this time."