The probe began "following a significant increase in the amount of spam appearing as 'spoofed emails' from AOL Mail addresses," the Internet company said in a statement.
Spoofing is a tactic used by hackers to make it appear that the message is from an email user known to the recipient in order to trick the recipient into opening an attachment which may contain malware, AOL noted.
"AOL's investigation is still underway, however, we have determined that there was unauthorized access to information regarding a significant number of user accounts," the statement said.
The company added that "we believe that spammers have used this contact information to send spoofed emails that appeared to come from roughly two percent of our email accounts."
AOL said it was working with "best-in-class external forensic experts and federal authorities to investigate this serious criminal activity."
The statement said it was not known whether the encryption on passwords or answers to security questions was broken, but that "as a precautionary measure, we nevertheless strongly encourage our users and employees to reset their passwords used for any AOL service.
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