The US Transportation Security Administration (TSA) is facing criticism for secretly tracking Americans on flights, US media report.
The "Quiet Skies" programme reportedly uses an unknown algorithm to flag flyers without any criminal record for surveillance on domestic flights.
Air marshals tasked with carrying out surveillance have pushed back against the programme, according to US media.
The US Transportation Security Administration denies any racial profiling and says it is a "practical" method.
[Source:BBC News]
