The FBI is investigating a hacking attack on JPMorgan Chase and at least one other bank, according to reports.
Bloomberg.com said the FBI was investigating an incident in which Russian hackers attacked the US financial system earlier this month in possible retaliation for US government-sponsored sanctions aimed at Russia.
The attack, Bloomberg said, led to the loss of sensitive data. Bloomberg cited security experts saying the attack appeared "far beyond the capability of ordinary criminal hackers".
The New York Times, also citing people familiar with the matter, said JPMorgan and at least four other companies were hit this month by what it described as coordinated attacks that siphoned off huge amounts of data, including checking current and savings account information.
In a statement, JPMorgan spokeswoman Trish Wexler said companies "of our size unfortunately experience cyber-attacks nearly every day. We have multiple layers of defence to counteract any threats and constantly monitor fraud levels."
Wexler did not confirm the reports and the FBI declined to comment.
The Wall Street Journal also cited unnamed sources in a less detailed report that called the attacks a "significant breach of corporate computer security".
JPMorgan is the largest US bank by assets.