Hackers are responsible for stealing customers’ names, account numbers, and contact information via their database, Citibank said in a statement on Thursday.

Citigroup Inc. discovered a data breach in their system in early May, but did not disclose any details at the time. As of now, the company states that customers’ Social Security numbers, dates of birth, card security codes, and expiration dates remain safe. They did not say when the attacks occurred and have since referred the incident to law enforcement.

The company told the AP in a statement they have implemented measures to prevent attacks of this nature in the future, and they are in the process of informing affected customers and replacing about 100,000 credit cards. The incident affected about 1 percent of the company’s 21 million North American customers. Citi’s debit cards have not been affected.

This attack is the latest in a series of hackings against large-name companies, including Gmail, Sony Corp., and Lockheed Martin Corp. However, financial institutions are still considered the primary targets for cybercrime.

In response to the attacks and questions of security, Wells Fargo & Co. said they were aware of the attacks at Citibank and that security remained the core of their mission.