The names and emails of customers of Citigroup Inc. and other large U.S. companies, as well as College Board students, were exposed in a massive and growing data breach after a computer hacker penetrated online marketer Epsilon Data Management LLC, Reuters reported.

In what could be one of the biggest such breaches in U.S. history, a diverse swath of companies that did business with Epsilon stepped forward over the weekend to warn customers that some of their electronic information could have been exposed, Reuters said.

Walgreen Co., TiVo Inc., Capital One Financial Corp. and HSN Inc. added their names to a list of targets that also includes some of the nation’s largest banks.

The names and electronic contacts of some students affiliated with the College Board — which represents about 5,900 colleges, universities and schools – could have been compromised, Reuters said.

No personal financial information such as credit card or Social Security numbers appeared to be exposed, according to statements from the companies and emails to customers.

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

Citigroup customer names and some credit card customers’ e-mail addresses — but no account information — were part of the data breach, the bank said.

The College Board, which administers the SAT admissions tests, warned students about the breach and asked them to be cautious about receiving “links or attachments from unknown third parties,” according to two emails reviewed by Reuters.