(Michael Riley)  Russian hackers attacked the U.S. financial system in mid-August, infiltrating and stealing data from JPMorgan Chase & Co. (JPM) 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, which occurred in mid-August, 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.

The sophistication of the attack and technical indicators extracted from the banks’ computers provide some evidence of a government link, but the trail is muddy enough that investigators are considering the possibility that it’s cyber criminals from Russia or even elsewhere in Eastern Europe. Other federal agencies, including the National Security Agency, are now aiding the investigation, a third person familiar with the probe said.

“The way the Russians do it, to the extent we can see into the process, is they encourage certain targets,” said James Lewis, director of the Strategic Technologies program at the Center for Strategic and International Studies inWashington. “The Russians typically keep open the options to do something more, and the question now is what would trigger that and what would our response be.”

Calls to JPMorgan for comment were not immediately returned.