New York Police Working on Technology to Detect Concealed Weapons For years, detectives trying to distinguish gun-carrying New Yorkers from others have had to rely on observations, street smarts and luck. A man with a gun on his hip might grab the front of his sport coat to keep it from flapping open and revealing the pistol. Someone getting out of a cab might hold tight to his side, to keep a weapon secure. But science is now promising to assist such human efforts. In a speech on Tuesday morning to the New York City Police Foundation, Police Commissioner Raymond W. Kelly said the department was working with the Defense Department to develop gun-scan technology "capable of detecting concealed firearms." The tool would operate as a sort of reverse infrared mapping tool by reading the energy people emit and pinpointing where that flow is blocked by some object, like a gun. "If something is obstructing the flow of that radiation - for example, a weapon - the device will highlight that object," Mr. Kelly said in his annual speech about the state of the Police Department. This technology has shown a great deal of promise as a way of detecting weapons without a physical search. Donna Lieberman, the executive director of the New York Civil Liberties Union, said that while the group yearned for solutions to gun violence, the proposal was worrisome. "We have no idea how this technology works, if it is effective, and what its error rate is," she said. If the N.Y.P.D. is moving forward with this, the public needs more information about this technology, how it works and the dangers it presents. The technology is still being tested at the police shooting range at Rodman's Neck, said Paul J. Browne, the department's chief spokesman. One issue is extending the range for identifying those with concealed arms. While it has been effective at distances of three to five meters, Mr. Browne said, "we're looking for it to be effective up to 25 meters," or around 82 feet. That would be a help to the police, who otherwise must rely on observation. Jack Maple, a former deputy police commissioner, wrote in his book, "The Crime Fighter: Putting the Bad Guys Out of Business": "I'd stand in front of a full-length mirror and study the way a gun looked under a jacket, over the shoulder, inside the waistband - anywhere on the body it could be hidden. On the job, I'd stop two or three people a day who were carrying concealed weapons.