Several US banks including JPMorgan Chase and at least four other US banks were hit by hackers in mid-August sparking a federal investigation.

The hackers infiltrated the networks of the banks, siphoning off gigabytes of data, including private information on employees and customers, in what security experts described as a sophisticated cyberattack.

The Federal Bureau of Investigation (FBI) was examining the case to see if it is retaliation for US sanctions against Moscow over its support of Ukraine’s secessionist rebels, Bloomberg reported citing multiple people familiar with the investigation.

But Bloomberg’s sources said there is not enough evidence at this time to prove that Russia’s government stood behind the breach.

The FBI said they were investigating the reports without confirming details at this point.

A JPMorgan Chase spokeswoman did not confirm the attacks, saying that "companies of our size unfortunately experience cyberattacks nearly every day. We have multiple layers of defense to counteract any threats and constantly monitor fraud levels."