WASHINGTON-—Former national security adviser Mike Flynn interacted with a graduate student with dual Russian and British nationalities at a 2014 U.K. security conference, a contact that came to the notice of U.S. intelligence but that Mr. Flynn, then the director of the Defense Intelligence Agency, didn’t disclose, according to people familiar with the matter.

Mr. Flynn met Svetlana Lokhova at the Cambridge Intelligence Seminar, a gathering of former intelligence officials hosted at Cambridge University, in February 2014....