Armed US air marshals have been secretly ‘staking out’ passengers at airports and on flights since March this year.
The US Transportation Security Administration (TSA) confirmed details of the programme, christened Quiet Skies, to members of the US Congress this week after the Boston Globe newspaper revealed the in-flight surveillance.
At the same time, the TSA denied reports that it plans to end security screening at some airports.
The TSA, which forms part of the US Department of Homeland Security, said the heightened surveillance began in March this year.
Passengers are selected for enhanced surveillance under a set of undisclosed criteria. But agency papers reveal teams of US air marshals document passengers’ behaviour, including whether they have a “cold penetrating stare” or “fidget” and use a computer or smartphone.
The TSA said about 5,000 US citizens were among the passengers monitored under the programme since March.
Almost 2.6 million passengers fly in and out of US airports daily.
A TSA spokesman said: “The programme analyses information on a passenger’s travel patterns and adds an additional line of defence to aviation security.
“It provides an ability to look at individuals who have exhibited patterns of travel not unlike what terrorists have exhibited in the past.
“Along with other data, it trips an alert that places [individuals] into a category where a federal air marshal might be on the flight.”
The spokesman added: “It’s a sensitive program. Putting too much information out about it provides an advantage to adversaries to potentially game the system.”
The TSA said the programme would continue.
However, the agency denied widespread reports that it is considering ending security screening at up to 150 smaller US airports.
In a statement, the TSA said: “There has been no decision to eliminate passenger screening at any federalised US airport.
“Every year TSA is asked to discuss potential operational efficiencies – this year is no different. Any potential operational changes would not take place without a risk assessment to ensure the security of the aviation system.”