While it’s never been as popular among evildoers as anthrax or other biological agents, ricin, the toxin believed to be contained in at least three letters sent to officials in Washington this week, including the president, is nasty and lethal in its own right. There’s no known antidote, it’s relatively easy to manufacture, and it's deadly in small quantities, rapidly destroying the functioning of the nervous system, with effects similar to those of sarin.

The U.S. government first investigated potential offensive uses of ricin, which can be rendered from a byproduct of castor oil, during World War I as a coating for bullets, and then during World War II for deployment in cluster bombs, but it was never weaponized on a mass scale as the military found other agents more effective. And while doses the size of a grain of salt can kill you, that’s still far bigger than the lethal dosage of agents like anthrax or the toxin that causes botulism.

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Dr. Leonard Cole, the director of the Program on Terror Medicine and Security at the Center for BioDefense at the University of Medicine and Dentistry of New Jersey, and the author of several books on bioterrorism, told Salon that while there’s never been “a wholesale successful use of ricin to kill a lot of people,” it does afford some benefits to a would-be terrorist.

First, it's relatively easy to produce with only “moderate sophistication.” Al-Qaida, for instance, reportedly investigated ricin after it was unable to produce more lethal biological agents. Stephen Morse, a professor of epidemiology at Columbia University who has edited a book on bioterrorism, told Salon that ricin is “a favorite of amateurs." Someone could even grow the castor beans themselves, he noted. However, most amateurs can't produce ricin in the high purity levels needed for a major attack, he said.

Secondly, it’s very stable, meaning it can be put in an envelope and jostled around, heated, X-rayed, etc., and survive the trip to its destination, Cole said.

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Indeed, this is hardly the first time people have tried or succeeded in using ricin to kill.

Perhaps the most famous victim of the toxin is Georgi Markov, a Bulgarian dissident writer who was killed in London in 1978 when a stranger -- presumably a Communist agent -- jabbed him with an umbrella that delivered a tiny ricin-laced pellet into his leg. A similar attempt was made on another Bulgarian dissident just weeks earlier in the Paris metro, but the capsule failed to deliver its lethal dose. Three years later, a similar assassination attempt was carried out on a double agent working for the CIA while he was grocery shopping in the suburbs of Washington, D.C. The pellet hit him in the kidney, however, and his body expelled it, treating it like a kidney stone, so he survived. Again the KGB was suspected.

The Spy Museum in Washington even has a replica of the famed "Bulgarian Umbrella."

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Beyond Cold War treachery, ricin has been popular among white supremacists and other such radical groups. “Because it is so readily available and easily prepared, ricin has been an attractive tool for domestic extremists and terrorists,” said Mark Pitcavage, the director of Investigative Research at the Anti-Defamation League, which tracks such groups.

“There have been several instances when known white supremacists have been trying to get this stuff and develop it for presumably unsavory practices,” Cole agreed.

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Recipes for making ricin can be found in materials available for purchase “at gun shows, survivalist expos, and across the Internet,” the ADL said, pointing to manuals such as, "The Poor Man's James Bond," "Ragnar's Action Encyclopedia of Practical Knowledge and Proven Techniques," "Silent Death," "Catalogue of Silent Tools of Justice" and "The Poisoner's Handbook."

In 1993, Canadian border agents stopped an American electrician named Thomas Lavy and found four guns, 20,000 rounds of ammunition, 13 pounds of gunpowder, neo-Nazi literature, $80,000 in cash, and 130 grams of ricin -- more than a quarter of a pound -- which he claimed he used to kill coyotes on his property. He killed himself in his holding cell just a few days later, so we'll never know what he really intended to do with the stockpile.

In 1994 and 1995, four men in Minnesota were arrested in an alleged plot to kill local sheriffs’ deputies, U.S. marshals and IRS agents with ricin and other weapons. The men were part of a radical anti-tax militia group called the Minnesota Patriots Council, which rejected the authority of the federal government and taxes. The plot was foiled when, after a dispute, one of the men’s wives took the contents of a coffee can to the FBI, which identified its contents as enough ricin to kill several hundred people.

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In 1998, three members of the North American Militia in Michigan were arrested on weapons and conspiracy charges after agents from the Bureau of Alcohol, Tobacco, and Firearms infiltrated the group and discovered an arsenal of weapons and a cooking-show-style video explaining how to make ricin.

In 1999, FBI agents arrested a man in Tampa, Fla., who had sent letters to local judges in Colorado, where he used to live, threatening to use ricin against them and warning that he could do with a briefcase full of ricin what Timothy McVeigh needed an entire truckload of explosives to accomplish.

In 2003 and 2004, letters sent to the White House, Senate office buildings and the Department of Transportation containing ricin were intercepted from someone calling themselves “Fallen Angel.” The government was thrown into chaos for a few days as authorities swept the buildings. A suspect was never apprehended, but interestingly, one of the letters was addressed from Chattanooga, Tenn. -- the same state where the ricin-laced letter sent to Sen. Roger Wicker this week originated.

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More recently, in 2011, Mary Morgan, a member of the Alaska Peacemakers Militia, was stopped at the U.S.-Canadian border and found with a firearm (which she was not allowed to have thanks to a felony conviction) and instructions about how to make ricin. The militia group had allegedly plotted to kill federal judges in the state and its leader was sentenced to 26 years in federal prison earlier this year.

Then, of course, there was the high-profile arrest of Georgia militia members that year who allegedly plotted to use ricin to kill massive numbers of civilians in metropolitan areas across the country.

It’s not just domestic extremists, however, who have investigated ricin's deadly potential. In 2003, British police arrested members of the Sunni extremist group Ansar al Islam for allegedly developing ricin for use in a terror attack in the country.

Beyond that, there are a smattering of routine homicides involving ricin poisoning, including scorned husbands, angry fathers and the like. Fairly comprehensive lists of incidents can be found here and here. There's no question the majority of them involve domestic radical groups, however.

As the 2003 ricin letters case show, along with that of the unabomber, it’s extremely difficult for authorities to track suspects who use the mail to deliver their deadly payload. “It would just seem by logic that if you’ve got three identified letters with ricin,” Cole said, “I think that there must be many, many more on the way, or in process. Or maybe they’ve already been sent and just haven’t been identified.”