Attacks systematically hit Bank of America, JPMorgan Chase, Wells Fargo, U.S. Bank and PNC Bank, CNNMoney reported Friday.
"The volume of traffic sent to these sites is frankly unprecedented," said Dmitri Alperovitch, co-founder of CrowdStrike, which is involved in an investigation of the cyberattacks.
The attacks were meant to overwhelm computer systems, with more requests for service than the systems can handle.
The attacks were enormous, Alperovitch said.
"It's 10 to 20 times the volume that we normally see, and twice the previous record for a denial-of-service attack."
The attacks are referred to as "denial-of-service" because a bank's computers are kept so busy denying service to unwanted requests that other parts of the system are slowed or the systems simply crash.
To pull off such an attack, "thousands of high-powered applications servers," send requests to the banks, CNNMoney reported.
The attacks did not involve theft of personal information, officials said.
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