Mathias was very troubled by the assassinations of President John F. Kennedy and his brother Senator Robert Kennedy. He was a member of the U.S. Senate, Select Committee to Study Governmental Operations with Respect to Intelligence Activities (the Church Committee). He played an important role in the Final Report, Book Five, "The Investigation of the Assassination of President John F. Kennedy : Performance of the Intelligence Agencies, 94th Congress, 2nd Session, 1976".