Al-Qaeda in the Arabian Peninisula is trying to develop bombs containing the deadly nerve toxin ricin, say US counterterror officials.

The New York Times:

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.

President Obama and his top national security aides were first briefed on the threat last year and have received periodic updates since then, top aides said. Senior American officials say there is no indication that a ricin attack is imminent, and some experts say the Qaeda affiliate is still struggling with how to deploy ricin as an effective weapon.

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.

But senior American officials say they are tracking the possibility of a threat very closely, given the Yemeni affiliate's proven ability to devise plots, including some thwarted only at the last minute: a bomb sewn into the underwear of a Nigerian man aboard a commercial jetliner to Detroit in December 2009, and printer cartridges packed with powerful explosives in cargo bound for Chicago 10 months later.

Making ricin is not difficult but deploying it as an effective WMD is another story. The explosives the article mentions would probably kill more people than the ricin they would be trying to disperse. And weaponizing the ricin, milling it down so that it is an extremely fine powder that could be used in aerosol devices requires not only technical expertise, but very expensive machines.

This is a serious threat but, as the article points out, no attack is imminent. What is most worrisome is that the imagination of these terrorists encompasses some very deadly scenarios - reason enough to keep up the pressure on them in Yemen with our drone strikes.