Customers of Verizon Communications had their email addresses exposed in a massive online data breach last week, according to an email to customers obtained by Reuters.

In what could be one of the biggest such attacks in U.S. history, a computer hacker penetrated the online marketer Epsilon, which controls the customer email databases for a broad swath of companies.

Customers of about 50 companies, from banks to retailers and hotels, had their names or email addresses exposed in the attack.

Verizon, the largest U.S. mobile telephone carrier, informed customers on Tuesday that it was part of the Epsilon data breach.

Epsilon has assured us that the information exposed was limited to email addresses, and that no other information about you or your account was exposed, Verizon said in an email to a customer sent on Tuesday evening.

The company did not immediately respond to requests for comment.

(Reporting by Maria Aspan in New York and Diane Bartz in Washington; Editing by Bernard Orr)

Verizon Communications Inc.