Law enforcement has arrested “an individual who is believed to have illegally obtained personal and account information of individuals,” including American Express members, according to a notification letter Amex posted online and also mailed to its customers.

The company said the information it received leads them to believe that customer data — including names, account numbers, expiration dates, four-digit security codes, mailing addresses, phone numbers, email addresses, mother’s maiden names, and possibly Social Security numbers — may have been stolen from other businesses. 

Amex is working with law enforcement in the ongoing investigation. It is offering customers a complimentary one-year membership of Experian’s ProtectMyIDTM Elite and will continue to monitor accounts for fraud activity.

The company urged customers to monitor their accounts closely for the next 12-24 months and to sign up to receive instant notifications of suspicious activity.