LOS ANGELES – A shadowy but well-organized hacker group in the Middle East has disrupted the electronic banking operations of America’s largest financial institutions in recent days, underscoring U.S. vulnerability to online terrorism.

A group identifying itself as Izz ad-Din al-Qassam Cyber Fighters attacked the websites of Wells Fargo, U.S. Bancorp and Bank of America. The strikes left customers temporarily unable to access their checking accounts, mortgages and other services.

The banks said account and personal information for their tens of millions of online and mobile customers were not compromised. Still, experts said the size and ferociousness of the attacks highlight the broader threat posed by electronic crime and the susceptibility of financial targets.