Q&A: What is ricin?
US authorities have intercepted a letter sent to Mississippi Senator, Roger Wicker, that preliminary tests showed contained the deadly poison ricin. The FBI and other agencies have launched an investigation.
What is ricin?
Ricin is a naturally occurring protein, found in the castor oil plant, which is highly toxic. It can be inhaled or ingested but it is most dangerous when it is injected.
How is it produced as a poison?
Various courts have heard detailed evidence of how ricin could be made. For obvious reasons, authorities are not keen on the full details being widely disseminated.
Suffice it to say that ricin is easily produced as an extraction from the beans of the castor oil plant.
How does it get into the body?
It can be delivered through the air, in an aerosol spray, or it can be injected or swallowed.
Deaths from eating castor plant beans are rare, partly because the skin of the bean is indigestible. But the pulp from eight chewed beans is considered potentially fatal for an adult. One to three beans chewed by a child can be fatal.
How potent is it?
It is 6,000 times more poisonous than cyanide.
70 micrograms or two millionths of an ounce, roughly equivalent to the weight of a single grain of salt, can be enough to kill an adult.
But terrorism expert Professor Paul Wilkinson told the BBC it was probably not possible to cause large numbers of casualties simply by spraying the substance into the air.
Ricin is estimated to be lethal at 70 micrograms for a person weighing 160lb if it is injected into the bloodstream.
Experts say using ricin to cause mass casualties would probably require using it as an additive to food and drink.
Such methods would require extensive prior research, development, operational planning and testing, and are beyond the means of most terror groups.
Is there any treatment?
The UK military is reported to have developed an antidote, although it has not yet been tested on humans.
An antidote developed by the US military has had some human testing.
But, generally, doctors can only treat a victim's symptoms and long-term organ damage is likely in survivors.
How would someone know if they were affected?
The first symptoms depend on how the person has been exposed to ricin but can include fever, stomach upsets and coughing.
If someone breathes the poison in, they could suffer serious lung damage eventually leading to heart failure.
If ricin gets into the digestive system, it can lead to irritation of the gut, gastroenteritis, bloody diarrhoea and vomiting.
It can also affect the central nervous system and cause seizures.
It can be several days before the most serious problems develop.
What other cases are there of ricin being used to poison?
It was used during the Cold War, in 1978, to kill Bulgarian dissident Georgi Markov in London.
A man with an adapted umbrella shot a small pellet containing ricin into Markov's leg on Waterloo Bridge.
Traces of ricin have been found in caves used by Islamist militants in Afghanistan.
In 2003, the Metropolitan Police raided a flat in north London and arrested six Algerian men who they claimed were manufacturing ricin as part of a plot to launch a poison attack on the London Underground.
No ricin was recovered and only one person was convicted over the plot - a man who had previously received a life sentence for killing a policeman during another raid.
In February 2004, Senate office buildings in in Washington DC were closed after ricin-contaminated mail was found.
Does ricin have any positive benefits?
Ricin is being studied to see if it could form the basis of cancer treatments.
It is hoped it could be used to destroy malignant cells within the body.