For more than a year, according to classified intelligence reports, Al Qaeda’s affiliate in Yemen has been making efforts to acquire large quantities of castor beans, which are required to produce ricin, a white, powdery toxin that is so deadly that just a speck can kill if it is inhaled or reaches the bloodstream.

Intelligence officials say they have collected evidence that Qaeda operatives are trying to move castor beans and processing agents to a hideaway in Shabwa Province, in one of Yemen’s rugged tribal areas controlled by insurgents. The officials say the evidence points to efforts to secretly concoct batches of the poison, pack them around small explosives, and then try to explode them in contained spaces, like a shopping mall, an airport or a subway station…

These officials also note that ricin’s utility as a weapon is limited because the substance loses its potency in dry, sunny conditions, and unlike many nerve agents, it is not easily absorbed through the skin. Yemen is a hot, dry country, posing an additional challenge to militants trying to produce ricin there.