Roger Wicker, a Mississippi Republican, received a letter containing ricin, a toxin derived from the castor oil plant.

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Dutschke, the affidavit stated, also said he'd never researched anything about ricin, a toxin derived from castor beans that has no known antidote.

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Four elderly men from the US state of Georgia have appeared in court charged with plotting to murder officials using explosives and the lethal toxin ricin.

BBC: Elderly men accused of US ricin plot

The FBI said the substance tested positive for ricin, a toxin derived from castor beans that has no known antidote.

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The FBI said the letters tested positive for ricin, a toxin derived from castor beans that has no known antidote.

CNN: Mississippi man arrested in ricin case

Three of four field tests conducted on the powder inside the envelope addressed to Wicker tested positive for a protein that later tests determined to be ricin, a lethal toxin, it says.

CNN: FBI confirms letters to Obama, others contained ricin

Senate Majority Leader Harry Reid of Nevada said Tuesday that the toxin might be ricin.

WSJ: Ricin Charges Are Dropped