“Iranian hackers have repeatedly attacked Bank of America Corp, JPMorgan Chase and Citigroup over the past year, as part of a broad cyber campaign targeting the United States, according to people familiar with the situation.

The attacks, which began in late 2011 and escalated this year, have primarily been “denial of service” campaigns that disrupted the banks’ websites and corporate networks by overwhelming them with incoming web traffic, said the sources.

Whether the hackers have been able to inflict more serious damage on computer networks or steal critical data is not yet known. The sources said there was evidence suggesting the hackers targeted the banks in retaliation for their enforcement of Western economic sanctions against Iran.”