From al-Qaeda to neo-Nazis, numerous hate groups have fantasized of pulling off a deadly terrorist attack using the highly lethal extract of the castor bean known as ricin. None has ever succeeded in carrying out such plans.

The four Georgians arrested this week in connection with an alleged terrorist plot may have been capable of advancing further than most amateur weaponeers, given their access to professional labs. (One had previously worked at the Centers for Disease Control, another for the Department of Agriculture.) But their chances for truly creating a weapon of mass destruction were tiny at best, biodefense experts say.

“Absolutely zero,” said Raymond Zilinskas, a microbiologist and expert on chemical and biological weapons.

The castor beans that are the source of ricin are relatively easy to obtain, and recipes for extracting the toxin are easily found on the Internet. But while a would-be terrorist could manufacture small batches of the poison in a basement or garage, the challenges involved in delivering lethal doses of ricin to large numbers of people are insurmountable for amateurs, said Zilinskas, director of the Chemical and Biological Weapons Nonproliferation Program at the James Martin Center for Nonproliferation Studies in Monterey, Calif.

“No one has done it, as far as we know,” he said. “It is beyond the capabilities of anyone except professional weapons scientists.”

Ricin is regarded as one of the world’s most toxic natural substances, so poisonous that a dose the size of a few grains of salt can kill. It has been effectively used in the past as an assassination weapon, most famously during the slaying of Bulgarian novelist and defector Georgi Markov, who was killed in 1978 by a suspected Bulgarian agent using a specially modified umbrella to inject the pellet of ricin into his victim.

But even governments with dedicated weapons laboratories have struggled to create a ricin weapon that can kill on a large scale. Former Iraqi president Saddam Hussein tried to do so during the late 1980s, but his scientists abandoned the effort after finding it was too difficult to convert ricin into a fine power or mist, according to records unearthed by U.N. weapons inspectors.

Justice Department officials say one of the men involved in the latest case got so far as obtaining castor beans, and the four -- all 65 or older, all alleged members of a far-right militia movement — had talked about using ricin in attacks on several U.S. cities.

Frederick Thomas, 73, one of the alleged co-conspirators, was quoted by court records as having boasted of his plans to kill, telling an undercover agent, “I’ve been to war, and I’ve taken life before, and I can do it again.”

Thomas’s wife, Charlotte, reached by phone by an Associated Press reporter, dismissed the alleged plot as “baloney.”