By Ian Bush

PHILADELPHIA (CBS) — Major banks are still trying to figure out what hit them in late September when their websites were knocked offline by cyber attacks. (Read Related Story)

There is new evidence that points to a sophisticated strike.

Middle Eastern hackers claim they’re behind the takedown of PNC, Bank of America, Wells Fargo, and other sites, but security experts say they must have had help.

“The attackers used very high-capacity, high-throughput servers that they compromised in advance,” says Ronen Kenig, director of security product marketing at Radware, the company investigating what happened. “And they used those servers to launch a very intense and massive attack.”

Kenig says someone infected data centers around the world with malware that can dodge antivirus efforts.

“Then the hackers will try to penetrate into the organization,” he says, “steal data, do some financial transactions, manipulations, and so on.”

There’s no evidence this attack caused any more than inconvenience for customers who couldn’t access their accounts, but Kenig warns banks to stay serious about security.