(Bloomberg) -- In July, Yahoo! Inc. received a report of a hacker claiming to have 280 million user account credentials for sale on the black market. An initial investigation found no evidence to back that up, according to a person familiar with the probe.

Claims like these are common nowadays. However, Yahoo decided to conduct a deeper, separate investigation and, piece by piece, the company slowly accumulated evidence of an even larger breach. Earlier this week, the person said there was enough evidence to tell Verizon Communications Inc., which had agreed to buy Yahoo’s web assets for $4.83 billion on July 25. The person asked for anonymity to discuss internal findings.

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