Facebook CEO Mark Zuckerberg has broken his silence over the Cambridge Analytica scandal, which saw the data of around 50 million users leaked.

Cambridge Analytica obtained the data by hiring psychology Aleksandr Kogan to create a personality quiz for Facebook users that harvested not only the data of the user taking the test, but that of all their Facebook friends.

In a rather lengthy post, Zuckerberg says that Facebook changed its rules in 2014 to “dramatically limit” the data apps could access. To wit, apps such as the one designed by Kogan would no longer be able to access user data without that user’s consent.

He says that when Facebook learned in 2015 that Kogan had handed the harvested data over to Cambridge Analytica, it demanded that both parties certify that they had deleted the data. While he says Kogan and Cambridge Analytica provided the necessary certifications, reports last weekend suggest Cambridge Analytica lied and retained the data for the duration of the 2016 US presidential race.

“This was a breach of trust between Kogan, Cambridge Analytica and Facebook,” Zuckerberg writes. “But it was also a breach of trust between Facebook and the people who share their data with us and expect us to protect it. We need to fix that.”

To that end, Zuckerberg says Facebook will be carrying out an investigation on all of the apps on its platform that had access to large amounts of information before the 2014 data gathering rule changes, and then auditing those that have exhibited any suspicious activity.

“We will ban any developer from our platform that does not agree to a thorough audit,” Zuckerberg writes. “And if we find developers that misused personally identifiable information, we will ban them and tell everyone affected by those apps.”

On top of that Zuckerberg says Facebook will be tightening its rules further on the information apps can access. Users will also see a tool for managing their privacy settings appear at the top of their news feed in the coming weeks. The tool already exists in the privacy settings menu, but Zuckerberg says he wants more users to be aware of it.