Eleven cities, including New York City, filed an amicus brief in the Southern District of New York this week to emphasize “the urgent need to regulate” so-called ghost guns, or gun parts that can be “quickly and easily transformed into fully operative guns.”

The cities’ brief comes after Everytown for Gun Safety and four cities, including Syracuse, sued the U.S. Department of Justice and the Bureau of Alcohol, Tobacco, Firearms and Explosives in August, arguing that ghost guns should be regulated as firearms.