Apparently in retaliation for U.S. sanctions against Russian banks, hackers in Russia have carried out an organized attack against America's largest banks, including JPMorgan Chase & Co., stealing gigabytes-worth of customer data.

Bloomberg News, which broke the story, reported that the FBI currently is aware of five banks involved in the intrusion, which apparently occurred when highly sophisticated hackers obtained access to the FIs' databases through an "unknown flaw" in their consumer-facing websites. European banks might also have been affected, a law enforcement source told Bloomberg.

Though the Putin administration has not been directly implicated in the theft, a Bloomberg commentator said that Russian hacking gangs appear to have close ties to officials. These organizations "do the bidding" of the government in exchange for the freedom to operate without interference from authorities — and with the understanding that the gangs will not victimize Russian entities, the commentator said.

The stolen information could be used to empty customers' bank accounts, but Chase said it has seen no abnormal account activity since the intrusion, whose timing corresponds to the enactment of sanctions against Russian banks, resulting from the country's interference in Ukraine, including its takeover of the valuable Crimean territory.

A law enforcement official told Bloomberg that the absence of fraudulent activity using the stolen data reinforces the idea that the attack was politically motivated.