Iranian hackers have repeatedly attacked Bank of America Corp, JPMorgan Chase & Co and Citigroup Inc 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.

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

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