AT&T customers should pay close attention to their account activity in the coming weeks after the company admitted that an employee illegally accessed users’ personal information. The data breach exposed Social Security information and driver’s license numbers of about 1,600 customers, and it’s not the first time this has happened.

The company said in a letter to consumers that the breach occurred in August and that the perpetrator had unauthorized access to AT&T account information. Michael Chiaramonte, AT&T’s director of finance billing operations, told customers that the infiltrator was able to access Customer Proprietary Network Information, which includes information about what services customers purchase, how they use those services and how much they pay.

“We recently determined that one of our employees violated our strict privacy and security guidelines by accessing your account without authorization in August 2014, and while doing so, would have been able to view and may have obtained your account information including social security number and driver’s license number,” Chiaramonte wrote.

AT&T confirmed in June of this year that it experienced a similar data breach, also from inside the company. While it’s not clear how many customers were affected in August, the security lapse earlier this year impacted an estimated 500 or more customers. AT&T is providing a free year of credit monitoring for affected customers.

“You may also want to consider contacting the major credit reporting agencies to place a fraud alert on your credit report, and to learn about identity theft programs offered by the Federal Trade Commission,” Chiaramonte’s letter went on. “To strengthen your account security, we recommend that if you currently have a passcode on your account, you change it. If you do not have a passcode on your account, we recommend you add one.”