Letters addressed to President Barack Obama and Senator Roger Wicker (R-Miss.) preliminarily tested positive for ricin, authorities said April 17. To help provide context to these incidents, START has compiled background information on the history of terrorism involving biological agents and toxins, particularly ricin. The START Background Report examines terrorist plots and attacks involving biological agents from 1990-2011, finding that terrorist organizations, cells and lone actors with political or ideological motives expressed interest in or plotted (22%), pursued acquisition of (44%), threatened (8%) or used (26%) 21 different biological agents in 74 separate incidents. Of these cases, 31 also involved a chemical agent. Based on open sources, from 1990-2011, a variety of perpetrators have been tied to 21 ideologically-motivated cases involving ricin, none of which are known to have caused injuries or casualties. The 21 cases are outlined within the report.
Pinson, Lauren, and Mila Johns, Gary Ackerman. 2013. "Ricin Letters Mailed to President and Senator." April. http://www.start.umd.edu/start/publications/br/STARTBackgroundReport_RicinMailings_April2013.pdf