The Federal Bureau of Investigation has opened an investigation into a possible security breach of AT&T Inc.'s website that exposed the email addresses of some owners of Apple Inc. iPad devices.

"The FBI is aware of these possible computer intrusions and has opened an investigation," said Katherine Schweit, an FBI spokeswoman. Ms. Schweit said the FBI began the investigation Thursday but wouldn't comment on what the bureau is looking at. "It's very early in the investigation," she added.

The...