The company, which had 100 million subscribers at the end of the third quarter, said it is advising less than 1 percent of its mobile customers that there was an attempt to obtain information about their accounts.
It said that the people involved appeared to have used "auto script" technology to see if AT&T telephone numbers were linked to online AT&T accounts.
Spokesman Mark Siegel said AT&T's "investigation is ongoing to determine the source or intent of the attempt to gather this information."
"In the meantime, out of an abundance of caution, we are advising the account holders involved," he said.
In its letter to customers AT&T warned them to be cause of emails or texts asking for sensitive information because there "may be an increased risk of fraudulent attempts to access" account information.
AT&T is seeking to buy T-Mobile USA, a unit of Deutsche Telekom.
(Reporting by Sinead Carew)