Here's a look at what you need to know about 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."
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.
Ricin can be produced easily and cheaply but analysts say as a weapon it is most effective on individuals.
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.
1980s - Some reports indicate that ricin may have possibly been used in the Iran-Iraq war.
March 20, 2003 - Ricin is found in a Paris train station, the Gare de Lyon.
February 2, 2004 - A powdery substance tentatively identified as ricin is found in Senator Bill Frist's mailroom in the Dirksen Senate office building.
February 3, 2004 - Senate Majority Leader Bill Frist says that tests have identified the substance as ricin. Federal authorities say they found no indication of international terrorism in the discovery and that a criminal investigation is under way. The full Senate convenes as usual, but the Dirksen, Hart and Russell Senate office buildings remain closed after the discovery.
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. Both letters are under federal investigation. The letter in Greenville was addressed to the Department of Transportation and demanded that changes in truckers' sleep/work schedules not be implemented.
July 28, 2004 - The FBI announces that three jars of baby food, in two separate incidents, have been contaminated with ground castor beans. The incidents took place in Irvine, California.
February 22, 2008 - A small amount of ricin, an anarchist handbook and firearms are found in a Las Vegas hotel room. The FBI is called in and later declares to media that it does not seem terroristic in nature.
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 14th due to respiratory problems.
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 sentence 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 questioned Levenderis, he said the substance was ant poison, but later admitted it was 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.
March 28, 2011 - Michael Crooker pleads guilty to one count of using the mail to threaten a federal prosecutor and one count of possessing the toxin ricin without obtaining required registration.
June 20, 2011 - Michael Crooker is sentenced to 15 years in prison.
November 1, 2011 - Prosecutors in Atlanta announce that federal agents have charged four Georgia men with plotting to attack government officials with explosives and ricin. The names of the accused are Dan Roberts, Frederick Thomas, Ray Adams and Samuel Crump.
April 10, 2012 - Dan Roberts and Frederick Thomas plead guilty to conspiring to obtain an unregistered explosive device and gun silencer. They are both sentenced to five years in prison.
April 16, 2013 - An envelope addressed to Senator Roger Wicker of Mississippi tests positive for ricin at the U.S. Capitol's off-site mail facility. A law enforcement source tells CNN that after the envelope tested positive in a first routine test, it was retested two more times, each time coming up positive. The letter is sent to the Army's biomedical research laboratory at Fort Detrick, Maryland for further testing.
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. Tests are being conducted to see if the letter contains ricin.
April 17, 2013 - The FBI arrests Paul Kevin Curtis at his home in Corinth, Mississippi on suspicion of sending tainted letters to President Obama and Senator Wicker.
April 18, 2013 - The Justice Department announces that Curtis is being charged with "knowingly depositing for conveyance in the mail and for delivery from any post office any letter, paper, writing or document containing threats to take the life of or to inflict bodily harm upon the President of the United States; and (2) knowingly depositing and causing to be delivered by the Postal Service according to the directions thereon, communications addressed to other persons, and containing a threat to injure the person of others."
April 23, 2013 - Curtis is released from federal custody. His attorney Christi McCoy says her client has been framed by someone who used several phrases Curtis likes to use on social media. "I do believe that someone who was familiar and is familiar with Kevin just simply took his personal information and did this to him," McCoy told CNN. "It is absolutely horrific that someone would do this."
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 suspected of containing ricin.
May 31, 2013 - The FBI confirms that "very low concentrations" of ricin were found on letters sent to President Obama, Mayor Bloomberg and the group Mayors Against Illegal Guns.
May 31, 2013 - Federal authorities search the home of a Texas man in connection with ricin-tainted letters that were mailed to President Obama and Mayor Bloomberg. The man's wife contacted authorities, saying she had found suspicious containers in their home.
June 3, 2013 - James Everett Dutschke is indicted in the mailing of ricin-laced letters to President Obama, Republican Sen. Roger Wicker of Mississippi and Lee County Judge Sadie Holland.
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. Richardson initially told authorities that her husband, Nathaniel Richardson, sent the letters. He was never charged.
June 27, 2013 - Shannon Richardson is indicted for sending letters containing ricin to President Obama and Mayor Bloomberg.
December 10, 2013 - Shannon Richardson pleads guilty to possession of a toxin for use as a weapon.
January 17, 2014 - A jury finds Ray Adams and Samuel Crump of Georgia guilty of plotting to make ricin.
January 17, 2014 - James Everett Dutschke pleads guilty to developing ricin and sending contaminated letters to President Obama and others.
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.
May 19, 2014 - James Everett Dutschke is sentenced to 25 years 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.
July 16, 2014 - Shannon Richardson is sentenced to 18 years in prison.
September 29, 2014 - Jeff Boyd Levenderis is sentenced to six years in prison.
November 14, 2014 - Samuel Crump and Ray Adams are sentenced to 10 years each in prison.