How Mr. Zuckerberg publicly addresses these problems in congressional hearings on Tuesday and Wednesday will be closely scrutinized. Facebook faces possible regulation and other changes amid a backlash over the lack of data privacy.
The Cambridge Analytica outcry was triggered after The New York Times and others reported last month that a quiz app made by Mr. Kogan had collected information on Facebook users. That information was then used by Cambridge Analytica to build psychological profiles of voters in the United States and others.
It is not clear whether the direct messages were among the data eventually provided to Cambridge Analytica. In an interview on Tuesday, Mr. Kogan told The Times that the private messages were harvested from a limited number of people, likely “a couple thousand.”
He said the messages were collected as part of research that he conducted at Cambridge University in 2013 and the first half of 2014, before he began working with Cambridge Analytica. The messages were collected for research into how people use emojis to convey emotions.
Mr. Kogan said the messages were kept securely in his university lab, known as the Cambridge Prosociality and Well-Being Lab, and access was restricted to a small group of people.
The message data “was obviously sensitive so we tried to be careful about who could access it,” Mr. Kogan said. He stressed that his Facebook app collected messages only from a “couple thousand” people who completed his questionnaire, not from their friends.
During Mr. Kogan’s later work for Cambridge Analytica, his Facebook app took data from people who took his questionnaire and from all their friends. But the data did not include private messages — it included only names, birth dates, locations and pages the users had liked, he said.