On September 10, 1897, Luzerne County Sheriff James Martin and 150 armed deputies fired on approximately 400 unarmed immigrant coal miners marching peacefully to support a newly formed United Mine Workers union at Lattimer, Pennsylvania. At least 19 miners, mostly Polish, Slovak, Lithuanian, and German, were killed-many shot in the back. Sheriff Martin and 73 deputies were arrested and tried for murder. Despite medical evidence of miners shot in the back and witness testimony contradicting the deputies' claim of self-defense, all defendants were acquitted. The massacre became a turning point for the UMW, resulting in over 10,000 new members.