(BLOOMBERG) — Russian hackers attacked the U.S. financial system in mid-August, infiltrating and stealing data from JPMorgan Chase & Co. and at least one other bank, an incident the FBI is investigating as a possible retaliation for government-sponsored sanctions, according to two people familiar with the probe.

The attack resulted in the loss of gigabytes of sensitive data and authorities are investigating whether recent infiltrations of major European banks using a similar vulnerability are also linked to the attack, one of the people said.

In one case, the hackers used a zero-day vulnerability in one of the banks’ websites, then plowed through layers of elaborate security to steal the data, a feat several security experts said appeared far beyond the capability of ordinary criminal hackers. The incidents occurred at a low point in relations between Russia and the West, as Russian troops continue to mass on the Ukrainian border and the West tightens sanctions aimed at crippling Russian companies, including some of the country’s most important banks.

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