For many living in a terror-spooked country, it might seem like a great government innovation: Use vans equipped with mobile X-ray units to scan vehicles at major sporting events, or even randomly, for bombs or contraband.

But news that the US is buying custom-made vans packed with something called backscatter X-ray capacity has riled privacy advocates and sparked internet worries about "feds radiating Americans."

"This really trips up the creep factor because it's one of those things that you sort of intrinsically think the government shouldn't be doing," says Vermont-based privacy expert Frederick Lane, author of "American Privacy." "But, legally, the issue is the boundary between the government's legitimate security interest and privacy expectations we enjoy in our cars."

American Science & Engineering, a Billerica, Mass.-company, tells Forbes it's sold more than 500 ZBVs, or Z Backscatter Vans, to US and foreign governments. The Department of Defense has bought the most for war zone use, but US law enforcement has also deployed the vans to search for bombs inside the US, according to Joe Reiss, a company spokesman, as quoted by Forbes.

On Tuesday, a counterterror operation snarled truck traffic on I-20 near Atlanta, where Department of Homeland Security teams used mobile X-ray technology to check the contents of truck trailers. Authorities said the inspections weren't prompted by any specific threat.

The mobile X-ray technology works by bouncing narrow X-ray streams off an object like a car and then analyzing the scatter rate of the returning rays. Operators can then locate less-dense objects that could be bodies or bombs.

Backscatter X-ray is already part of an ongoing national debate about its use in so-called full body scanners being deployed in many US airports. In that case, US officials have said they will not store or share the images and will use masking technology to avoid revealing details of the human body. Nevertheless, information security advocates have filed suit to stop their deployment, citing concerns about privacy.

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.

"Regardless of where you fall on the spectrum of national security … you have to be realistic that this is another way in which the government is capturing information they may lose control over," says Mr. Lane. "I just have some real problems with the idea of even beginning a campaign of rolling surveillance of American citizens, which is what this essentially is."