While data mining works in some cases, such as for detecting credit card fraud, it is a totally unproved technique for uncovering terrorist plots, said Bruce Schneier, chief technology officer at managed service provider BT Counterpane in Mountain View, Calif.

"It's just plain silly," said Schneier, who was one of the 16 security experts who added their signatures to the coalition of privacy groups that filed a comment with the DHS earlier this week. "There isn't enough data to find patterns, and the instances of what you are looking for are so small that the false alarms will kill you," he said.

Others who have called for review of the DHS system include the American Civil Liberties Union, which filed formal comments with the DHS earlier this week. In its comments, the ACLU argued that the ATS would put the government into the "business of creating 'security ratings' for millions of its own citizens." Such a course of action had the potential to "alter the relationship between the state and the individual," the ACLU said.