The attack, which was first reported by BusinessWeek, affected less than 1 percent of AT&T's more than 100 million customers, according to a spokesman.
"We recently detected an organized and systematic attempt to obtain information on a number of AT&T customer accounts, including yours," AT&T said in an email to customers. "We do not believe that the perpetrators of this attack obtained access to your online account or any of the information contained in that account."
AT&T told BusinessWeek that the attackers used auto script technology to "determine whether AT&T telephone numbers were linked to online AT&T accounts," a spokesman told BusinessWeek.
"No accounts were breached and our investigation is ongoing to determine the source or intent of the attempt to gather this information," the spokesman said.
In related report on the AT&T breach, a spokesperson said the attackers' motives are unknown.
"We do not know the intent, but we are concerned they may attempt to deceive our customers by sending them unsolicited texts or emails claiming to be from AT&T and requesting sensitive personal information like Social Security numbers or passwords," the spokesperson said.
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