Enlarge / CEO Mark Zuckerberg speaks at Facebook’s 2016 “F8” conference. (credit: Facebook)

After days of silence, Facebook CEO Mark Zuckerberg has responded to the controversy over the 2014 leak of private Facebook user data to a firm that went on to do political consulting work for the Donald Trump campaign in 2016.

Cambridge Analytica got the data by paying a psychology professor, Aleksandr Kogan, to create a Facebook personality quiz that harvested data not only about its own users but also about users’ friends. Kogan amassed data from around 50 million users and turned it over to Cambridge.

Zuckerberg says that when Facebook learned about this transfer in 2015, it got Kogan and Cambridge to certify that they had deleted the data. But media reports this weekend suggested that Cambridge had lied and retained the data throughout the 2016 presidential campaign.