Yahoo took its time investigating massive security breach

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SAN FRANCISCO Yahoo detected evidence that a hacker had broken into its computer network at least 18 months before launching an investigation that discovered personal information had been stolen from about 500 million user accounts.

The timeline outlined in a regulatory filing raises further questions about why it took Yahoo so long to realize the severity of its security breakdown. It also could provide Verizon Communications with reason to revise or terminate its $4.8 billion deal to buy Yahoo's online services.

Yahoo disclosed the size of the breach seven weeks ago. At that time, Yahoo traced its findings to an inquiry opened in late July.

But Yahoo's regulatory filing late Wednesday acknowledged the company first became aware of the hack in late 2014. It is now investigating how much was known then.