Security experts say expanding the X-ray technology for use on American streets is a powerful counterterror strategy. They also point out the images do not not offer the kind of detail that would be embarrasing to anyone. Moreover, law enforcement already has broad search-and-seizure powers on public highways, where a search warrant is often not needed for officers to instigate a physical search.

But others worry that radiating Americans without their knowledge is evidence of gradually eroding constitutional protections in the post-9/11 age.