As a bioterrorism agent, ricin has the advantage of being easily made and highly potent. But there have been few fatal cases in the past 50 years, and there is little precise information about the substance’s effects on human beings.

Ricin is not a microbe. It does not grow inside the body and can’t be passed from person to person. It is a toxin produced by the castor bean plant. When the beans are crushed for oil, the compound is left behind in the mashed material, of which more than a million tons is produced around the world each year.

“It is a plant that grows wild throughout much of North America. You can buy the seeds online,” said Jennifer A. Oakes, a physician and expert in ricin poisoning at Albany Medical College. “It doesn’t take much to get a fatal dose. Somebody could do this in their house if they are motivated to.”

Ricin’s best-known victim is Georgi Markov, a Bulgarian journalist who was stabbed by an umbrella on a London street in 1978. The umbrella’s tip injected a tiny metal capsule containing ricin into Markov’s leg. He died three days later.

Apart from him, the only other ricin fatalities in the past 50 years have been a few suicides and accidental poisonings, usually after castor beans were eaten but at least once by injecting a crude extract. A person needs to take about 1,000 times as much ricin by mouth as by other routes to get a fatal dose.

Unlike nerve agents and botulinum toxin, which disrupt nerve transmission and can cause death in minutes, ricin acts slowly. It stops the synthesis of proteins in cells, killing them over hours or days. A person dies of multi-organ failure as cells break down and fluid and essential electrolytes are lost.

There is no antidote to ricin poisoning. At least two vaccines against the poison are under development.

Monkey experiments have shown that inhaling powdered ricin — the material found in the recent letters — causes bleeding in the lungs and suffocation within about three days. Injected ricin causes symptoms similar to severe infection, including fever, nausea and shock. Eating ricin causes abdominal pain, vomiting and diarrhea within about six hours.

Clinical data about ricin poisoning, however, are sketchy because cases are so rare.

While the raw material is easy to get, purifying ricin from the mashed beans isn’t easy.

“Somebody would have to have a chemistry background to know how to do that extraction,” Oakes said. “But given what’s on the Internet, I can’t imagine that the information is really difficult to obtain.”

Even harder would be grinding the purified powder to a size that stays in the lungs when inhaled. Particles too large are caught in the nose or upper airways. Particles too small are inhaled and then exhaled. Monkey studies suggest that particles one to two microns in diameter are the most dangerous.

Ricin has been detected in threatening letters and suspicious places. In 2003, a letter containing the substance and a threat to poison the public water supply was found at a mail-processing center in Greenville, S.C. In 2008, ricin was found in a room at an extended-stay motel in Nevada, along with firearms and what authorities termed an “anarchist-type textbook.”