According to the Transportation Security Administration (TSA), that has admitted the Quiet Skies, the program has monitored about 5,000 U.S. citizens on domestic flights in recent months.

Quiet Skies was criticized by privacy advocates because the authorities have begun monitoring U.S. citizens that aren’t suspected of a crime or of involvement in terrorist organizations.

The domestic surveillance program aims at collecting extensive information about the movements of the citizens and their behaviour.

“The previously undisclosed program, called ‘Quiet Skies,’” specifically targets travelers who “are not under investigation by any agency and are not in the Terrorist Screening Data Base,” states a bulletin issued in March by the TSA.

The Agency 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 or phone numbers associated to terrorism suspects could trigger monitoring.

Passengers remain on the Quiet Skies watch list “for up to 90 days or three encounters, whichever comes first, after entering the United States,” according to the TSA. Travelers are not notified when they have been added to the watch list.

Every day about 40 to 50 people on domestic flights are selected under the Quiet Skies program and on average, air marshals follow and monitor about 35 of them.