The shooting, which left four dead and 17 injured, is just the latest spurt of mass violence made possible by the small 3D-printed part known as a “conversion device,” “Glock switch” or “auto sear.”

Authorities are offering a reward of up to $100,000 in exchange for more information about multiple shooters who opened fire in Birmingham, Alabama, over the weekend in a mass shooting that left four people dead and 17 others injured.