Kelly has called the practice a lifesaving tool that deters criminals from taking illegal weapons in public.

WSJ: NYPD program patrols inside private buildings

"You get a sense of why we're so hopeful about this tool, " Mr. Kelly told the audience, which was mostly members of the New York City Police Foundation, a nonprofit group that raises money for the department.

WSJ: Police Test New Scanner

New York Mayor Michael Bloomberg and Police Commissioner Raymond Kelly say it is a necessary, life-saving, crime-fighting tool that helps keep illegal guns off the street and has helped New York reach all-time crime lows.

WSJ: Trial over NYPD stop-and-frisk tactic set to begin