Citigroup Inc and Capital One Financial Corp were part of a large data breach that exposed the names and e-mail addresses of many large companies' customers, including those at some of the nation's largest banks.

The online marketer Epsilon, a unit of Alliance Data Systems Corp, said on Friday that a person outside the company hacked into some of its clients' customer files. The vendor sends more than 40 billion e-mail ads and offers annually, usually to people who register for a company's website or give their e-mail addresses while shopping.

Citigroup customer names and some credit card customers' e-mail addresses -- but no account information -- were exposed as part of the data breach, the third-largest U.S. bank said on Saturday.

Capital One also said the Epsilon hacker accessed its customer e-mail addresses, but no personally identifiable information.

JPMorgan Chase & Co, the second-largest U.S. bank, and Kroger Co, the biggest U.S. supermarket operator, said on Friday that some customers had their names and e-mail addresses exposed as part of the data breach.

Citigroup announced that it had been affected on Saturday evening. Spokesman Sean Kevelighan said the bank started informing its customers of the breach on Friday through a link on its website.

(Reporting by Maria Aspan, additional reporting by Jonathan Spicer, editing by Maureen Bavdek)

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