US domestic passengers who have no criminal record are being followed under an airport security program.
Questions are being raised over a new American airport security program where US air marshals shadow the movements of ordinary domestic passengers and report suspicious behaviour, such as repeated trips to the bathroom, to the Transport Security Administration (TSA).
The 'Quiet Skies' program, first reported by the Boston Globe, identifies travellers who could pose a threat but have no criminal record and aren't on any terrorist watch list.
According to the Globe, undercover air marshals have been following passengers at an airport and on a flight, documenting behaviour like excessive fidgeting, perspiration, face touching and the use of a smartphone.
Launched in 2010, an unknown algorithm is reportedly used to flag the travellers with the goal of thwarting threats to commercial aircraft posed by "unknown or partially known terrorists".
However, marshals tasked with carrying out the surveillance have criticised the program, while US lawmakers have demanded answers from the TSA about the controversial program.
The US Marshals have complained the costly and time-consuming task inhibits their ability to do more vital law enforcement work.
Hugh Handeyside, a lawyer with the American Civil Liberties Union's National Security Project, said the surveillance raises a number of constitutional questions.
"These concerns and the need for transparency are all the more acute because of TSA's track record of using unreliable and unscientific techniques to screen and monitor travellers who have done nothing wrong," he told the BBC.
Aviation security specialist Chris Cubbage, executive editor of Australian Security Magazine, whose background includes the Australian Crime Commission and the Western Australian Police, was not as critical of the program "as long as it's been managed appropriately."
"If you can link someone's digital signature, to their behaviour signature and then with their travel then you may be on to something," Mr Cubbage told SBS News.
He says, however, that the program must be targeted and not just based on monitoring human behaviours, otherwise it would be like trying to find a "needle in a haystack".
"When you've got Air Marshals across every flight just randomly looking out for people who are sweating that's when you fall into the danger of wasting money, and people obviously get anxious flying anyway so monitoring behavioural traits alone is insufficient," said Mr Cubbage.
"Anyone can be nervous for any reason," he added.