I have looked at history and unions have engaged in violence and terrorism in America.
Worst terrorist strikes in the United States
http://www.johnstonsarchive.net/terrorism/wrjp255us.htmldate attack and location fatalities injuries
11 Sep 2001 crashing of hijacked planes into World Trade Center, New York City, New York, Pentagon in Alexandria, Virginia, and site in Pennsylvania 2,993 8,900
19 Apr 1995 truck bombing of federal building, Oklahoma City, Oklahoma 169 675
16 Sep 1920 bombing near bank in New York City, New York 38 300
*1 Oct 1910 bombing at newspaper building in Los Angeles, California 20 20
5 Nov 2009 shooting at Soldier Readiness Center at Foot Hood, Texas 13 43
*4 May 1886 bombing and riot at Haymarket Square rally in Chicago, Illinois 12 60
29 Dec 1975 bombing at airport in New York City, New York 11 75
*22 Jul 1916 bombing at parade in San Francisco, California 10 40
26 Feb 1993 truck bombing of World Trade Center, New York City, New York 6 1,040
8 Jan 2011 shooting attack at political event, Tucson, Arizona 6 13
*=Unions
1910 Los Angeles Times Bombing
http://en.wikipedia.org/wiki/Los_Angeles_Times_bombing1886 Haymarket Riot and Bombing
http://en.wikipedia.org/wiki/Haymarket_affair1916 Preparedness Day Bombing
http://en.wikipedia.org/wiki/Preparedness_Day_Bombing