August 28, 2014 - The FBI is investigating cyber-attacks on US banks, including JP Morgan Chase, by Russian hackers, according to press reports.

Citing two sources, Bloomberg says that gigabytes of sensitive data was stolen in the sophisticated attack, which happened in the last month. A zero-day vulnerability in one bank website was exploited by the hackers, who then managed to get past layers of security to get to the data.

Unsourced reports suggest that the mystery assailants burrowed so deeply into JPMorgan's computer systems that they may even have been able to alter or delete bank account records.

At least five US banks are thought to have been targeted. Similar attacks have been seen recently on European banks, and authorities are looking into whether these are linked.

The nature of the hit suggests that there may have been government involvement at a time when tensions between the US and Russia bubbled up over the Ukraine crisis, says the Bloomberg report.

However, US authorities think that the attack could also just be down to Russian cyber-crooks, although JP Morgan has not seen a noticeable rise in fraud since the attack, according to the Wall Street Journal.

The assault has also been downplayed by the Financial Services Information Sharing and Analysis Center, which shares information on cyber threats among the largest US banks.

"There are no credible threats posed to the financial services sector at this time," the group said in an email posted to member firms in the wake of Thursday's news reports.