A number of United States banks, including JPMorgan Chase and at least four others, were struck by hackers in a series of coordinated attacks this month, according to four people familiar with a continuing investigation into the crimes.
The hackers infiltrated the networks of the banks, siphoning off gigabytes of data,including checking and savings account information, in what security experts described as a sophisticated cyber attack.
The motivation and origin of the attacks are not yet clear, according to investigators. The F.B.I. is involved in the investigation, and in the past few weeks a number of security firms have been brought in to conduct forensic studies of the penetrated computer networks.
According to two other people briefed on the matter, hackers infiltrated the computer networks of some banks and stole checking and savings account information from clients. It was not clear whether the attacks were financially motivated, or if they were collecting intelligence as part of an espionage effort.
JPMorgan has not seen any increased fraud levels, one person familiar with the situation said.
"Companies of our size unfortunately experience cyber attacks nearly every day," said Patricia Wexler, a JPMorgan spokeswoman. "We have multiple layers of defense to counteract any threats and constantly monitor fraud levels."
J.Peter Donald, an F.B.I. spokesman in New York, declined to comment, citing the current investigation.
The intrusions were first reported by Bloomberg, which indicated that they were the work of Russian hackers. But security experts and government officials said they had not yet made that conclusion.
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Earlier this year, iSight Partners, a security firm in Dallas that provides intelligence on online threats, warned companies that they should be prepared for cyber attacks from Russia in retaliation for Western economic sanctions.
But Adam Meyers, the head of threat intelligence at CrowdStrike, a security firm that works with banks, said that it would be "premature" to suggest the attacks were motivated by sanctions. Mr. Meyers said he could not speak specifically about any of his company's clients.
Russian hackers began a month long online assault on Estonia in 2007 that nearly crippled the Baltic nation, after Estonian government workers moved a Soviet-era war memorial from the Estonian capital.
Still, security experts say that the stealthy nature of the recent attacks suggests that their motivation was not political.