In an appearance on CBS' "Face the Nation," Kelly said the new technique uses terahertz technology.
"Basically, everyone emits what they call terahertz radiation. And if that radiation is blocked by something, such as a weapon, you can see the outline of it. Now we've tested it. We just received the latest prototype. And it's very encouraging," he said.
The prototype, Kelly said, is too big for deployment at this point.
"But we're getting there. It's sort of like the cellphone was 20 years ago," he said.
Kelly said the police department is working with its attorneys to see that the technology is "appropriately used" in case of civil liberties groups who may say it is too invasive of people's privacy.
"We understand there are Fourth Amendment [reasonable search] issues here," he said. "So we want to get everybody on board before this goes to any widespread use."