A suspicious substance found on a letter addressed to President Obama is likely ricin poison. An almost identical letter was sent to Sen. Roger Wicker (R-MS), which did test positive for the deadly poison leading officials to suspect the same of this one.
Ricin, from the castor oil plant Ricinus communis, is a highly toxic, naturally occurring protein. A dose as small as a few grains of table salt can kill an adult human. The LD50 of ricin is around 22 micrograms per kilogram (1.78 mg for an average adult, around 1⁄228 of a standard aspirin tablet/0.4 g gross) in humans if exposure is from injection or inhalation. Oral exposure to ricin is far less toxic and a lethal dose can be up to 20–30 milligrams per kilogram.
The Georgia Militiamen were arrested in 2011 for planning domestic terror attacks, including bombing Federal buildings and mass poisonings using homemade ricin.
From a report by The Southern Poverty Law Center:
Another member of the group, Samuel J. Crump, 68, allegedly said at a September meeting that he wanted to make 10 pounds of deadly ricin powder and disperse in a series of cities. He said that it could be blown out the windows of a car traveling down an interstate without harming the plotters. An affidavit said that Crump just last week said he was going to begin shelling castor beans, from which ricin is manufactured. Another alleged plotter, Ray H. Adams, 65, allegedly said he had a formula for making ricin and knew ways to get the needed ingredients. “I’d say the first ones that need to die is the ones in the government buildings,” Adams allegedly said during an April 2011 meeting.
Watch the preliminary reports, courtesy of NBC: