Epsilon sent 6.5 billion e-mail marketing messages in 2009, but the company also runs loyalty programs for Citi and Chase credit card users, and the kind of information stored in its databases could be extremely valuable to criminals looking to steal banking information in phishing attacks.

Epsilon told Chase that none of its customers' financial information was compromised, the bank said Friday in a press release.

Kroger has posted a document about the incident.