Here's a look at ricin, a natural, highly toxic compound that is extracted from castor beans.
About one million tons of castor beans are processed every year. Ricin is part of the waste from the beans when castor oil is made. Sometimes this is called "mash."
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Less than a pinpoint can kill a human if ingested, injected, or inhaled. There is no known cure.
Ricin can be used in powder, pellet, mist or acid form.
If injected, ricin causes immediate death of the muscles and lymph nodes near the site of the injection. Failure of the major organs and death usually follow.
If ingested it causes nausea, vomiting and internal bleeding of the stomach and intestines, followed by failure of the liver, spleen and kidneys, and death by collapse of the circulatory system.
If a castor bean is swallowed whole without damage to the seed coat, it will probably pass harmlessly through the system. But if the coat is chewed or broken, then swallowed, the poison will enter the body.
1978 - Bulgarian dissident Georgi Markov is shot with a ricin-tipped dart fired from an umbrella while waiting for a bus in London. He dies four days later.
February 2, 2004 - Ricin is found in Senator Bill Frist's mailroom in the Dirksen Senate office building.
February 3, 2004 - Law enforcement sources say that a letter addressed to the White House was intercepted in November 2003 after it was found to contain ricin. Ricin was also found in a letter in October 2003 at a postal handling facility in Greenville, South Carolina. The letter in Greenville was addressed to the Department of Transportation and demanded that changes in truckers' sleep/work schedules not be implemented.
February 22, 2008 - A small amount of ricin and firearms are found in a Las Vegas hotel room.
March 1, 2008 - Roger Von Bergendorff, a California resident, is named as the occupant of the Las Vegas hotel room. Bergendorff had been rushed to the hospital on February 14 due to respiratory problems.
April 16, 2008 - Bergendorff is charged with possession of a biological toxin, possession of unregistered firearms and possession of firearms not identified by serial number.
April 28, 2008 - Thomas Tholen of Utah, Bergendorff's cousin, pleads not guilty on charges he knew about the creation of ricin and did not report it. Investigators believe Bergendorff's ricin was created in Tholen's basement.
August 4, 2008 - Bergendorff pleads guilty to possession of a biological toxin and possession of unregistered firearms.
October 22, 2008 - Tholen receives two years' probation and is ordered to pay a fine of $500 for knowing his cousin produced and carried ricin.
November 17, 2008 - Bergendorff is sentenced to three and a half years in federal prison and fined $7,500.
January 28, 2011 - Jeff Boyd Levenderis is arrested after a coffee can containing ricin is found in his foreclosed home in Coventry Township, Ohio. When the FBI questions Levenderis, he says the substance is ant poison, but later admits it is ricin that he made 10 years earlier.
February 1, 2011 - A federal grand jury indicts Levenderis on one count of possessing a biological toxin and one count of making false statements.
April 16, 2013 - An envelope addressed to Senator Roger Wicker of Mississippi tests positive for ricin at the US Capitol's off-site mail facility.
April 17, 2013 - The Secret Service announces that a letter addressed to President Barack Obama, containing a "suspicious substance," was intercepted at the White House's off-site mail facility on April 16. Later in the day the FBI confirms that the letter tested positive for ricin.
April 17, 2013 - A law enforcement source tells CNN that the letters to President Obama and Senator Wicker contain the same language and read, "To see a wrong and not expose it, is to become a silent partner to its continuance." Both letters are signed, "I am KC and I approve this message."
April 17, 2013 - Lee County, Mississippi, Sheriff Jim Johnson says that a letter received by Judge Sadie Holland has notable similarities to letters sent to President Obama and Senator Wicker.
May 22, 2013 - Matthew Ryan Buquet is arrested in Spokane, Washington, and suspected of sending ricin-laced letters threatening federal Judge Fred Van Sickle.
May 29, 2013 - Officials intercept a letter sent to President Obama that is similar to letters sent to New York City Mayor Michael Bloomberg and the group Mayors Against Illegal Guns. The letters are postmarked Shreveport, Louisiana, and are found to contain "very low concentrations" of ricin.
June 3, 2013 - Dutschke is indicted in the mailing of ricin-laced letters to President Obama, Republican Sen. Wicker of Mississippi and Lee County Judge Holland. He later pleads guilty and is sentenced to 25 years in prison.
June 7, 2013 - Shannon Richardson, an actress who had a minor role in the TV series "The Walking Dead," is arrested and charged in connection with ricin-tainted letters that were mailed to President Obama and Mayor Bloomberg.
June 27, 2013 - Richardson is indicted for sending letters containing ricin to President Obama and Mayor Bloomberg.
March 21, 2014 - Georgetown University student, Daniel Harry Milzman, is arrested and charged with possessing a biological toxin after admitting he made ricin in his dorm room. Milzman is later sentenced to one year and one day in prison.
June 4, 2014 - Jeff Boyd Levenderis is convicted of possessing ricin for use as a weapon and also possessing ricin and making false statements to agents of the FBI. He is later sentenced to six years in prison.
November 30, 2017 - Seventy-year-old Betty Miller is arrested and charged with unregistered possession of a select agent. Miller tells an FBI agent that she was making ricin in her home and testing its effectiveness on other residents of the Wake Robin continuing care retirement community.
May 18, 2018 - Miller pleads guilty to possessing ricin.
October 2, 2018 - Envelopes suspected of containing ricin are addressed to President Donald Trump and three top government officials, FBI Director Chris Wray, Secretary of Defense James Mattis and chief of naval operations Adm. John Richardson. The envelopes sent to the Pentagon are intercepted at the Pentagon mail facility and the envelope addressed to Trump never enters the White House.
October 18, 2018 - The Justice Department announces a seven-count indictment against Allen, including an allegation that he "knowingly threatened to use a biological agent and toxin, specifically ricin, as a weapon." Allen pleads not guilty to all charges.