Aleksandr Kogan took private messages sent from and received by users who downloaded his app, This Is Your Digital Life.
The app that gathered private information later improperly obtained by the political consulting firm Cambridge Analytica also collected some users' private messages, The Guardian reported on Friday.
This Is Your Digital Life, a quiz app developed by a University of Cambridge academic named Aleksandr Kogan, pulled in both incoming and outgoing messages from several thousand accounts of people who downloaded the app.
The number of people who had messages taken is a small portion of the estimated 87 million users whose data was exposed, but it represents a much more intrusive collection than the page likes, birthdays, locations, and personality traits and so forth that were taken from other profiles.
Kogan denied handing over that information to Cambridge Analytica; it is unclear exactly what data the firm obtained and how it was used.
Facebook and Britain's Information Commissioner's Office are still investigating the situation. The fallout from the scandal precipitated Facebook CEO Mark Zuckerberg's appearance before three US congressional committees this week.