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WATCH: ‘It was clearly a mistake to believe’ Cambridge Analytica, Zuckerberg says

In the first of two scheduled congressional hearings with Facebook CEO Mark Zuckerberg, Sen. Bill Nelson asked the 33-year-old executive why the company didn’t notify the estimated 87 million users whose data was improperly used by Cambridge Analytica.

Zuckerberg said Tuesday that Facebook had demanded that Cambridge Analytica delete and stop using any user’s personal information after the company learned of the firm’s data-mining through a third-party app developer.

WATCH: Facebook CEO Mark Zuckerberg testifies before Senate committee

“When we heard back from Cambridge Analytica that they had told us they weren’t using the data and had deleted [the app], we considered it a closed case,” Zuckerberg told the Florida senator. “In retrospect, that was clearly a mistake,” he said.

Zuckerberg also said he didn’t notify the Federal Trade Commission of the data transgression because the company considered the matter a closed case.