The TSA has monitored about 5,000 U.S. citizens on domestic flights in recent months. The program targets travelers who are not under investigation by any agency and are not in the Terrorist Screening Data Base. Travelers are not notified when they have been added to the Quiet Skies watch list. Air marshals follow and monitor about 35 of those selected under the program every day. The TSA is monitoring individuals who have spent a certain amount of time in specific countries, who have visited those counties within a certain period of time, or that have made a reservation which includes email addresses.”]
Source: https://securityaffairs.co/wordpress/75263/digital-id/quiet-skies-surveillance-us.html