The New York Police Department already has the long arm of the law. Now it gets X-ray eyes.

NYPD Commissioner Ray Kelly says officers are in the advanced stages of testing a futuristic new weapon in their war against gun-toting residents: a device that can detect the presence of hidden metal from a distance.

The device measures terahertz, the natural energy emitted by people and objects. "If something is obstructing the flow of that radiation, for example a weapon, the device will highlight that object," Kelly said in a speech Wednesday.

The boxy machine can be hidden in a vehicle, allowing officers to discreetly scan their target. The device is being seen as a possible alternative to the NYPD's reliance on the controversial tactic of stop-and-frisk, where officers routinely search large numbers of men in high-crime areas.

"We still have a number of trials to run before we can determine how best to deploy this technology. But we're very pleased with the progress we've made over the past year," Kelly said.