American counterterrorism officials are concerned that Al Qaeda’s affiliate in Yemen, "the most dangerous regional arm of Al Qaeda," is trying to produce explosives for attacks against the United States, the New York Times reports. And it turns out that Al Qaeda means to pack these small explosives with the poison ricin, which it has been trying to produce by gathering large quantities of castor beans, which are required to produce the poison. This is why this is such terrifying news:

What is ricin? Ricin, a white, powdery toxin, is so deadly that just a speck can kill if it is inhaled or reaches the bloodstream. From the Center for Disease Control and Prevention website:

Ricin works by getting inside the cells of a person’s body and preventing the cells from making the proteins they need. Without the proteins, cells die. Eventually this is harmful to the whole body, and death may occur... Death from ricin poisoning could take place within 36 to 72 hours of exposure, depending on the route of exposure (inhalation, ingestion, or injection) and the dose received.

The Telegraph adds that:

Ricin is so poisonous that inhalation of a few minute grains is enough to kill an adult. It was used in deadly incidents including the 1978 assassination in London of Bulgarian dissident Georgi Markov and, in 2002, was the subject of a bioterrorism scare in the UK capital...

What they plan to do with it? Intelligence officials said that the evidence points to Al Qaeda's plans to leave these ricin-packed explosives to explode in contained spaces, like a shopping mall, an airport or a subway station. But the Times notes that this would not amount to the threat posed by, say, 9/11:

A ricin-dispersing bomb detonated in a major subway system or in a mall or at a major airport would not result in mass destruction on the scale of the attacks on Sept. 11, 2001, counterterrorism specialists said.

How close are they? Here is where reports differ. Al Qaeda’s arm in Yemen has openly discussed using ricin against the United States: “Brothers with less experience in the fields of microbiology or chemistry, as long as they possess basic scientific knowledge, would be able to develop other poisons such as ricin or cyanide,” the organization posted to its online English-language journal last fall. However, the Times also indicates 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."

Nonetheless, Michael E. Leiter, who retired recently as director of the National Counterterrorism Center, said at a security conference last month. “It’s not hard to develop ricin.”