AT&T Inc., reaching out to iPad users Sunday to explain why their email addresses were released last week, blamed the incident on "computer hackers" who "maliciously exploited" an attempt by the carrier to speed the process of logging in to its website.

The comments were the harshest yet by the carrier, which apologized for the security lapse and said it would cooperate with any efforts to investigate or prosecute the breach.

"AT&T...