The pinpoint search: how super-accurate surveillance technology threatens our privacy.

• Author: Sanchez, Julian
• Position: Cover story

ANYONE WOULD CONSIDER it a stroke of bad luck to be pulled over for driving six miles per hour over the speed limit, but Roy Caballes had an additional reason to curse his ill fortune. On the November afternoon in 1998 when an Illinois state trooper stopped him, Caballes was carrying 282 pounds of marijuana in his trunk. At first it looked like he'd get off with just a warning. Then another officer pulled up and swept his car with one of the most advanced pieces of technology then available to law enforcement: a drug-sniffing dog named Krott. The pooch uncovered the dope.

Caballes thought the cops didn't have a legitimate reason to bring in Krott, and he fought the search. In 2003 the Illinois Supreme Court ruled that the officers had indeed violated the Fourth Amendment by transforming the traffic stop into a drug investigation without probable cause, or even the weaker "reasonable suspicion." But in the 2005 decision Illinois v. Caballes, the U.S. Supreme Court ruled that the dog sniff could not have rendered an otherwise lawful traffic stop unconstitutional unless the dog sniff itself violated Caballes' "constitutionally protected interest in privacy." The Court concluded it did not, citing a 1983 decision in which it ruled that, because a dog sniff reveals only the presence of contraband in which there is no "reasonable expectation of privacy," it isn't a "search" at all. The Supremes sent the case back to Illinois, and Caballes ended up with a 12-year prison sentence.

The dog sniff that caught Caballes is just one crude, old-fashioned example of the search technologies available to law enforcement. A new wave of advanced surveillance tools is capable of detecting not just drugs but weapons, explosives, and illicit computer files, potentially flying under the Fourth Amendment's radar all the while. A handheld scanner picks up stray particles of cocaine on a car during a routine traffic stop. Is that a search? A high-tech camera detects the gun one pedestrian is carrying under his jacket. Is that a search? A forensic analyst finds a single image of child pornography on a computer server containing thousands of files owned by hundreds of users, without ever seeing any other private information. Is that a search?

In a nation whose reams of regulations make almost everyone guilty of some violation at some point, Americans have grown accustomed to getting away with minor transgressions: the occasional joint or downloaded movie or high-speed dash to the airport. For at least some crimes, though, the expectation that our peccadilloes will slip through the cracks may soon be outdated. The new style of noninvasive but deeply revealing detection--call them "pinpoint searches"--will require rapid adjustments in both legal rules and social mores.

Anatomically Correct Searches

The original pinpoint search, the drug dog's sniff, has built-in limits. A German shepherd is a cumbersome piece of biotechnology, making suspicionless sweeps during routine traffic stops the exception rather than the rule. But chemists and engineers are developing a variety of electronic sniffers that are competing to make Fido's schnoz obsolete.

DrugWipes, for example, are small, swab-tipped devices. Wipe the tip along a surface, or a sample of sweat or saliva, and in two to five minutes a simple indicator window reveals whether drug residue is present. Manufactured by the German firm Securetec, DrugWipes have been used by more than 2,000 law enforcement agencies in the U.S. since the late 1990s, and they're increasingly popular among schools and private employers as well.

DrugWipes have limitations: They're single-use devices, and while the basic model is inexpensive (less than $10 per unit), each picks up only one specific type of drug residue. Even with a relatively low per-unit price, the cost of sweeping a school or a parking lot can mount quickly. For more versatility, cops can turn to General Electric's VaporTracer, a seven-pound handheld particle sniffer that can test for a wide range of drugs and explosives in only a few seconds.

The VaporTracer and its nonportable cousin, the Itemiser, are already used in airports to scan luggage for explosives. With a price tag of $25,000 to $30,000, the VaporTracer is unlikely to become standard issue for beat cops in the near future. (G.E. estimates that about 4,000 are in use worldwide, most for explosive detection.) But researchers are developing ever faster, cheaper, and more sensitive electronic noses.

Among the technologies in the offing is the desorption electrospray ionization scanner. It uses charged droplets to lift particles from a surface and into a mass spectrometer, which can break down and analyze the components of any substance down to the molecular level. It's currently a desktop-sized machine, but its creators, a team of researchers at Purdue University, hope to develop a portable version that can fit in a backpack within a few years. The Purdue team's head, Graham Cooks, guesses such a device might cost about $4,000. That's not exactly cheap, but it's thousands of dollars less than a well-trained drug dog costs.

Meanwhile, scientists at Georgia Tech have developed prototype scanning technologies based on a penny-sized surface acoustic wave chip, which works by measuring disturbances in sound waves as they pass across small quartz crystals. This "dog on a chip" sensor is coated with a thin layer of cloned antibody proteins that bond to a specific molecule, such as cocaine or TNT. The sound waves passing through that sensor can then be compared with an uncoated control crystal: Differences in the waves mean the chip has picked up trace amounts--as little as a few trillionths of a gram--of the target substance.

Handheld scanners aren't the only possible application for such sniffer chips. Metro stations in Washington, D.C., have been fitted with fixed chemical weapon detectors, meant to give advance warning in case of a terrorist attack. Sensors with a range of a few feet could be combined with surveillance cameras to pinpoint passengers who might be worth extra scrutiny.

Police can use new devices to hunt not just for tiny traces of contraband but for larger objects. Millimeter wave (MMW) radiation is all around us. You're emitting it even as you read this article. More important, you're emitting it through your clothes, making it an ideal way to scan for hidden objects that distort or block those waves, whether they're made of metal, ceramic, plastic, or some other composite material--and without any of the health concerns associated with X-rays.

The Federal Aviation Administration began funding MMW research...