Some of Citigroup Inc's banking sites went down for about half an hour on Wednesday, making its online banking system temporarily inaccessible to customers, some of whom were housebound in the wake of giant storm Sandy.
Bank spokesman Andrew Brent said the outage also affected applications for mobile devices such as the iPad, but that all sites and apps had been restored to service.
It was not clear what caused the problem, which occurred after the New York-based bank moved some of its operations to backup facilities. Its lower Manhattan buildings used for trading and investment banking were closed on Wednesday as the city struggled to recover from the historic storm.
Brent declined to say if the outage was caused by fallout from Sandy or other issues.
Citi, Bank of America Corp , JPMorgan Chase & Co and other U.S. banks have repeatedly been attacked by Iranian hackers over the past year as part of a broad cyber campaign targeting the United States, according to people familiar with the situation.
The industry kept that campaign a secret until last month after a group that calls itself the Cyber Fighters of Izz ad-din Al Qassam began claiming credit for a spree of high-profile attacks that caused temporary outages at major U.S. banks.
The hackers behind the attacks have used sophisticated and diverse tools that point to a carefully coordinated campaign, security researchers have told Reuters.