That prospect has sparked fears that the databases authorities are building could someday be used for monitoring political rallies, sporting events or even busy downtown areas. Whatever the security benefits — especially at a time when terrorism remains a serious threat — the mass accumulation of location data on individuals could chill free speech or the right to assemble, civil libertarians say.
"As a society, do we want to have total surveillance? Do we want to give the government the ability to identify individuals wherever they are . . . without any immediate probable cause?" asked Laura Donohue, a Georgetown University law professor who has studied government facial databases. "A police state is exactly what this turns into if everybody who drives has to lodge their information with the police."
Facial-recognition systems analyze a person's features — such as the shape of eyes, the curl of earlobes, the width of noses — to produce a digital "template" that can be quickly compared with other faces in a database.
The images must be reasonably clear, though newer software allows technicians to sharpen blurry images, bolster faint lighting or make a three-dimensional model of a face that can be rotated to ease comparisons against pictures taken from odd angles.
For the state officials issuing driver's licenses, the technology has been effective at detecting fraud. As millions of images are compared, the software typically reveals the identities of hundreds or thousands of people who may have more than one driver's license.
When searches are made for criminal investigations, typically a photo called a "probe" is compared against existing images in a database. The analytical software returns a selection of potential matches, though their accuracy can vary dramatically. A probe image of a middle-aged white man, for example, could produce a possible match with a 20-something African-American woman with similarly shaped eyes and lips. Many systems include filters that allow searchers to specify race, sex and a range of possible ages for a suspect.