Ten demonstrators were killed by police bullets during the "Little Steel Strike" of 1937. When several smaller steelmakers, including Republic Steel, refused to follow the lead of U.S. Steel (Big Steel) by signing a union contract, a strike was called by the Steel Workers Organizing Committee (SWOC) of the Congress of Industrial Organizations (CIO).
As a show of support, hundreds of SWOC sympathizers from all around Chicago gathered on Memorial Day at Sam's Place, where the SWOC had its strike headquarters. After a round of speeches, the crowd began a march across the prairie and toward the Republic Steel mill. They were stopped midway by a formation of Chicago police. While demonstrators in front were arguing for their right to proceed, police fired into the crowd and pursued the people as they fled. Mollie West, a Typographical Union Local 16 member and a youthful demonstrator at the time, still recalls the command addressed to her: "Get off the field, or I'll put a bullet in your back."
http://www.kentlaw.edu/ilhs/memorial.htmFurther details:
http://www.kentlaw.edu/ilhs/republic.htmGraphic photos:
http://www.neiu.edu/~reseller/memdaymasscr.htmLaFollette Commission testimony:
http://historymatters.gmu.edu/d/138/pnorman