Chiaramonte explained that the employee could have accessed information such as driver's license, Social Security and Customer Propriety Network Information (CPNI), which refers to the cutomers' subscription information regarding the company's services.
"We recently determined that one of our employees violated our strict privacy and security guidelines by accessing your account without authorization in August 2014," Chiaramonte wrote.
"And while doing so, would have been able to view and may have obtained your account information including your social security number and driver's license," he added. "Additionally, while accessing your account, the employee would have been able to view your Customer Propriety Network Information."
According to Mark Siegel, AT&T's executive director of media relations, the employee was able to access the account of around 1,600 of the company's customers, according to ZDNet.
Since the CPNI of customers were also compromised, the company notified federal law enforcement agencies regarding the security breach.
Although the employee's identity was not revealed to the public, Siegel noted that the person has been fired from the company, PC World reported.
"This individual no longer works at AT&T, and we are directly contacting the limited number of affected customers," Siegel said.
To ensure the protection of their various accounts, Chiaramonte advised customers to contact credit reporting agencies and change their passcodes.
"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," he wrote.
"To strengthen your account security, we recommend that if you currently have a passcode on your account, you change it," he added. If you do not have a passcode on your account, we recommend you add one."