Bulgarian dissident and correspondent Georgi Markov is shown in this undated handout picture.
Ricin: A Deadly Weapon
It was known as the "umbrella death" -- the 1978 assassination of Bulgarian dissident, novelist and journalist Georgi Markov. He was killed in London by the deadly poison ricin, delivered into the back of his thigh by a man with an umbrella.
Markov, a reporter who worked at times with Radio Free Europe, the BBC and other news organizations during his career, had angered officials in the Soviet KGB and the Bulgarian secret service. Less than a week after the man jabbed him with the umbrella near London’s Waterloo Bridge, Markov was dead from multiple failure of his internal organs.
Tests later showed the mysterious man had used the umbrella to inject a tiny pellet containing Ricin into Markov’s leg. His assassination became the most infamous example of a using ricin as a murder weapon.
The toxic substance is now being investigated by top U.S. officials because letters believed laced with the deadly poison have turned up addressed to President Barack Obama and members of Congress.
What is ricin?
Ricin is a highly toxic poison that is naturally produced when castor beans are processed for their oil. The toxin is part of the waste from making the oil and can be a powder, a mist or a pellet, according to the U.S. Centers for Disease Control and Prevention.
If inhaled, injected or ingested, an extremely small dose of ricin – often only a drop -- can quickly produce symptoms and kill a person within 36 to 48 hours due to respiratory or circulatory system failure. There is no known cure.
The major symptoms depend on the means of exposure and include breathing difficulties, fever, coughing and nausea when ricin is inhaled.
If significant amounts of the poison are swallowed, vomiting, diarrhea, severe dehydration and possible seizures can follow. Over several days, liver, spleen and kidney failure could result in death.
Ricin symptoms are not contagious and the CDC says it would take "a deliberate act" to make the substance and "use it to poison people." Ricin is not typically absorbed through the skin and unintentional exposure is unlikely. Medical experts point out it is a less potent killer than anthrax.
Other cases
Officials dressed in protective suits prepare to enter the closed Russell Senate Office Building on Capitol Hill, to retreive mail that could be contaminated, Feb. 4, 2004.
Letters tainted with anthrax appeared in post offices, newsrooms and U.S. congressional offices after the September 11, 2001 terror attacks, resulting in five deaths and several illnesses.
Ricin previously turned up in a U.S. Senate mailroom in 2004, forcing authorities to temporarily shut down two Senate office buildings.
In 2003, Islamist militants were arrested in Britain for planning to put ricin in food on a British military base.
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