An additional 14 million users were affected more deeply, having additional details taken related to their profiles, such as their recent search history, gender, educational background, geolocation data, birth dates, and lists of people and pages they follow.

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Facebook said last month that it detected the attack when it noticed an uptick in user activity. An investigation soon found that the activity was linked to the theft of security codes that, under normal circumstances, allow Facebook users to navigate away from the site while remaining logged in.

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The bugs that allowed the attack to occur gave hackers the ability to effectively take over Facebook accounts on a widespread basis, Facebook said when it disclosed the breach. The attackers began with a relatively small number of accounts that they directly controlled, exploiting flaws in the platform’s “View As” feature to gain access to other users' profiles. (The “View As” feature is designed to allow users to view their own profiles as though they are somebody else.)

Facebook said it is cooperating with federal and other authorities on its investigation but said the FBI had advised the company not to discuss who may be behind the attack.

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What may have motivated the attackers is still unclear; despite mounting concerns about election security as U.S. officials count down to a highly contested midterm election, Facebook said there was no indication the hack was specifically related to the U.S. electoral process.

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“We don’t have a specific indication as to the intention of the hackers,” said Guy Rosen, Facebook’s vice president of product management.

Although the hackers could have used the flaw to steal information belonging to other, third-party apps that use Facebook as a login method, Facebook said Friday that no outside apps appear to have been affected. Neither Instagram nor WhatsApp appears to have been compromised, the company added. Facebook Messenger was also unaffected.

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The 29 million affected users, along with 1 million whose security tokens were taken but did not appear to have their data stolen, will be receiving customized messages from Facebook identifying specifically which types of information on their profiles, if any, were involved in the breach. Facebook executives told reporters Friday that the company will also try to reach affected users who have since deleted their Facebook profiles.

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Facebook has also established a Web page that will inform users who are logged in whether their accounts were affected.

User messages could have been exposed in one specific use case, officials said. If an affected user had been the administrator of a Facebook page, and the page had received a message from another user, that message may have been compromised, Facebook said.

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Facebook’s disclosure puts the company under even greater pressure as policymakers have taken the company to task over its approach to user privacy and data.

“The update from Facebook today is significant now that Facebook has confirmed that the personal data of millions of users was taken by the perpetrators of the attack," said Ireland’s Data Protection Commission — the watchdog agency charged with monitoring compliance with the European Union’s new data privacy law. It said it was continuing an investigation into the breach.

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The Federal Trade Commission — which Facebook said it is cooperating with — didn’t immediately respond to a request for comment.

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The spotlight on tech companies intensified further this week as Google said that half a million accounts on its Google+ social networking service could have had information leaked as a result of a software bug. The admission prompted lawmakers to demand answers from the company and call for an FTC investigation.