The DOS Office of Investigations and Counterintelligence conducts a robust counterintelligence program designed to deter, detect, and neutralize the efforts of foreign intelligence services targeting Department of State personnel, facilities, and diplomatic missions worldwide. The office's counterintelligence division conducts aggressive counterintelligence inquires and counterespionage investigations with other U.S. Government agencies. All counterespionage investigations are conducted in close coordination with the FBI in accordance with their statutory mandate to prosecute instances or allegations of suspected espionage.
The DOS Diplomatic Security Service (DSS) is a large, multifaceted law-enforcement and security organization within the U.S. Department of State, protecting U.S. personnel, property, and information around the world, as well as visiting foreign dignitaries, in order to provide a secure environment for the conduct of U.S. foreign policy.
Video on former Deputy Director of the Office of Counterintelligence, Robert Booth, and his book "Spyscape." May 2020.