BBC News Online looks at the deadly poison ricin and how it could be used as a weapon.
A thousandth of a gram can kill
What is it?: Ricin is a natural toxin, or poison, derived from the castor oil plant.
Effects and symptoms: As little as a thousandth of a gram can kill an adult. It can be deadly if eaten, inhaled or injected. Ricin poisoning will create flu-like symptoms within a few hours, including fever, vomiting and coughing. Ultimately it can cause the lungs, liver, kidneys and immune system to fail. Death can occur within three days. Ricin poisoning is not infectious.
Appearance: Ricin can be extracted from the beans of the castor plant (see image B) and can take on a liquid, crystal or powder form.
Use as a weapon: Although highly toxic, ricin is probably more suited to the assassination of individuals rather than use on a battlefield or a mass attack on a city. Iraq experimented with using ricin in artillery shells, but was unable to make it work. As the toxin can be fatal if inhaled it could be delivered to victims in an aerosol form.
History: Ricin was found in Afghanistan after the fall of the Taleban Government. The deadly poison was used to kill Bulgarian dissident Georgi Markov in London in 1978. A tiny amount was placed in a pellet (see image A) and injected into his leg with a modified umbrella.
Production: The castor oil plant is available across the globe and the toxin is stable, which together make the manufacture of ricin a relatively straightforward process.
Protection: The use of gas masks would prevent against the inhalation of ricin. There is no known antidote.