The Liberator’s designs were downloaded more than 100,000 times in just two days before federal officials demanded their removal in May. The risk, officials said, is not that street criminals will use printed weapons in their day-to-day operations. Rather, ATF officials are concerned about individuals slipping plastic guns past metal detectors and into schools, sporting events or government offices.

Depending on the sensitivity of a metal detector, a bullet might not be enough to set it off. Only X-ray machines could spot the handgun itself, and in many public places they aren’t available.

“There are ways that this can potentially create a huge problem for the American public,” said Richard Marianos, an assistant ATF director.