Almost every person entering and leaving the United States by air, sea or land is assessed, based on ATS's analysis of their travel records.
Other data is also analysed, including items such as where a person is from, how they paid for tickets, their motor vehicle records, past one-way travel, seating preferences and the kinds of meals they ordered.
Travellers are not allowed to see or directly challenge the risk assessments, which the government intends to keep on file for 40 years.
However, the information can be given to foreign states and, under certain circumstances, private contractors.
"Never before in American history has our government gotten into the business of creating mass 'risk assessment' ratings of its own citizens"
Barry Steinhardt, lawyer for the American Civil Liberties Union
"It is simply incredible that the Bush administration is willing to share this sensitive information with foreign governments and even private employers, while refusing to allow US citizens to see or challenge their own terror scores," Leahy said.
This system "highlights the danger of government use of technology to conduct widespread surveillance of our daily lives without proper safeguards for privacy," he said.
Business representatives also expressed concern at the procedures.
"I have never seen anything as egregious as this," said Kevin Mitchell, president of the Business Travel Coalition.
He said it was "evidence of what can happen when there isn't proper oversight and accountability."
Privacy advocates said the data-mining programme was unprecedented.
"Never before in American history has
our government gotten into the business of creating mass 'risk assessment' ratings of its own citizens," said Barry Steinhardt, a lawyer for the American Civil Liberties Union.
|"Never before in American history has our government gotten into the business of creating mass 'risk assessment' ratings of its own citizens"|
Barry Steinhardt, a lawyer for the American Civil Liberties Union
Steinhardt added the ACLU was stunned that the programme has been undertaken "with virtually no opportunity for the public to evaluate or comment on it."
The homeland security department says the nation's ability to spot criminals and other security threats "would be critically impaired without access to this data."
The targeting system operates from an unmarked building in Virginia.
Investigators from the homeland security's customs and border protection agency analyse information from multiple sources, not just ATS.
Based on all the information available to them, federal agents turn back about 45 foreigners, apparent criminals, a day at US borders, accrording to Bill Anthony, a homeland security's customs and border protection spokesman.
He could not say how many were spotted by ATS.