Controversy over the GT 200 has played out in recent months in Thailand, where the army has said it will continue to use them even though testing by government scientists found them far less effective than specially trained dogs. “I respect the scientific tests, but at this stage there is no banning order by the government, so the army will continue to use it,” Gen. Anupong Paochinda, the Thai Army chief, told reporters.

Human Rights Watch issued a statement in February calling on the Thai government to stop arresting people based on evidence gathered using the GT 200, which it said “performs worse than a roll of the dice.”

Informed that Mexico was using the same unit, the human rights group said Friday: “It’s troubling that Mexico is using this ‘magic wand’ technology given the serious doubts that exist about its reliability. And if people are actually being arrested and charged solely on the basis of its readings, that would be outrageous.”

Promotional materials on the Internet describe the GT 200 as a high-tech unit that enables law enforcement agencies to search large areas quickly. Using special cards provided by the manufacturer, the detector can supposedly detect all types of narcotics and explosives by homing in on their molecules from afar.

The device is so sensitive, the manufacturer says, that it can detect not just stockpiles of illegal drugs but people who have used cocaine or heroin as far back as two weeks before.

After the critical reviews in Thailand, Global Technical released a statement on its Web site defending the detector. “We can say that previous tests carried out by independent bodies, and the experience of the large number of users of this product all over the world, confirms that the GT 200 is effective and because of this, we would ask that you treat with caution any reports to the contrary,” the company said.

In Culiacán, a city in Sinaloa State where Mexican drug traffickers have a strong presence, the military showed off the GT 200 in December. Canvassing a residential neighborhood, soldiers walked up and down the street with a GT 200 waiting for the antenna to point toward a suspicious residence. There were no discoveries.