Skip to Content

Drive-by Gun Scans Prompt Privacy Questions

Police hope terahertz-scanning devices can be more effective than patting people down, but civil rights groups are wary.
January 24, 2012

Police in New York City are trying to develop something like x-ray vision, raising questions about the legal expectation for privacy in public places.

The NYPD recently announced that it’s working on a mobile scanner designed to detect concealed weapons on people from up to 75 feet away. The department won’t say when the devices might hit the street or how much they will cost, but Police Commissioner Raymond W. Kelly has already praised them as “a cutting-edge effort to deter gun violence.”

The police department is developing the device with the U.S. Department of Defense. The device measures terahertz waves—which are naturally emitted by people and objects and pass through materials such as clothes, which is how the scanner can reveal guns or other hidden objects. The current model can scan from up to about 15 feet away; Kelly hopes to increase the range to 75 feet before mounting the device on police vehicles.

City officials tout the technology as an alternative to NYPD’s controversial “stop and frisk” technique, in which officers stop people on the street for questioning—often patting them down in the process. Opponents of the practice say it unfairly targets minorities. Blacks and Hispanics accounted for 87 percent of the roughly 600,000 people stopped by the NYPD in 2010, according to the Center for Constitutional Rights, a civil rights organization based in New York City.

“I think this is a positive development,” says Richard Cardinale, a New York attorney who represents plaintiffs in police misconduct cases. “The biggest benefit of stop and frisk is that police are getting guns off the street and saving lives. If you have this new way to detect guns, people don’t have to suffer the indignity of being searched for no reason.”

Not everyone is ready to embrace the new technology, however. “There are serious Fourth Amendment problems raised by the use of this technology,” says Michael Price, counsel for the Brennan Center for Justice’s Liberty and National Security Program. “The idea that police could mount a scanner on a car and take images of everyone within a certain range is troubling.”

The Fourth Amendment guarantees individuals the right to be free from “unreasonable searches and seizures.” Generally, the law allows cops to stop and frisk a person if the officer has “reasonable suspicion” that the person is committing or about to commit a crime. The threshold is lower than the “probable cause” required for police to search a person’s car, get a warrant to search a home, or make an arrest.

Cardinale says scanning a person for weapons isn’t a search at all, likening the technology to traffic light cameras and speed guns, neither of which requires reasonable suspicion in order to be used.

The question of whether a certain technique is a search or frisk under the law largely turns on the subject’s privacy expectation. Courts generally require police to obtain a warrant before entering a home because, even in tiny New York City apartments, people expect a modicum of privacy there. In 2001, the Supreme Court ruled that police use of thermal imaging devices to check homes for the telltale heat produced by high-intensity lamps used in indoor marijuana-growing operations constitutes a search under the Fourth Amendment and is “presumptively unreasonable” without a warrant.

Even on the street, where a person has a lower privacy expectation, the law requires some minimal basis—reasonable suspicion—for stopping a person and searching him. That includes scanning for weapons, according to Price. “The Fourth Amendment doesn’t vanish when you leave your house.”

Kelly says that, no matter how extensively the devices are used, they won’t replace typical, old-fashioned police work. As he described the scanning project in a speech this month, he added: “We aren’t relying on technology alone to achieve our vision of the future.”

Keep Reading

Most Popular

Large language models can do jaw-dropping things. But nobody knows exactly why.

And that's a problem. Figuring it out is one of the biggest scientific puzzles of our time and a crucial step towards controlling more powerful future models.

The problem with plug-in hybrids? Their drivers.

Plug-in hybrids are often sold as a transition to EVs, but new data from Europe shows we’re still underestimating the emissions they produce.

Google DeepMind’s new generative model makes Super Mario–like games from scratch

Genie learns how to control games by watching hours and hours of video. It could help train next-gen robots too.

How scientists traced a mysterious covid case back to six toilets

When wastewater surveillance turns into a hunt for a single infected individual, the ethics get tricky.

Stay connected

Illustration by Rose Wong

Get the latest updates from
MIT Technology Review

Discover special offers, top stories, upcoming events, and more.

Thank you for submitting your email!

Explore more newsletters

It looks like something went wrong.

We’re having trouble saving your preferences. Try refreshing this page and updating them one more time. If you continue to get this message, reach out to us at with a list of newsletters you’d like to receive.