Customers of Wells Fargo, JPMorgan, Bank of America, and other institutions briefly lost access to their online accounts amid an attack by a Middle East-based hacker group—even after the group warned banks of the impending attack.

Account information wasn't compromised, the banks say, but the attacks have experts worried. "The banks put a lot of effort into cyber security. But they're so desirable as a target, even with all that effort they still have problems," notes an analyst.

JPMorgan Chase, Bank of America, and Citigroup saw attacks last week; Wells Fargo was hit Sept. 25, US Bank on Sept 26 and PNC is facing a threat today, September 27. Banks are working with federal authorities against the attacks; one banker says execs and officials have teamed up in a banking "war room."

A group called Izz ad-Din al-Qassam Cyber Fighters claimed credit for the attacks; Izz ad-Din al-Qassam is Hamas' military branch. The group says it will keep up the assault until the anti-Islam video “The Innocence of Muslims” is removed from the Internet. An expert, however, says the group has been launching attacks since the beginning of the year, before the appearance of the video, Newser reported.