Bioterror back, but panic is not

But ricin is also relatively easy to make. The anthrax that was sent through the US mail was of such a pure strain that investigators have focused their attention on scientists with specific training in biological weapons programs. Indeed, to this day, the FBI still hasn't been able to re-create the kind of anthrax that was used in the 2001 attacks.

"We never had an incident like this before," says FBI spokesman Bill Carter. "We had to start from the beginning."

Ricin, on the other hand, is derived from the mash left from the extraction of castor oil from the bean of the castor plant, and can be made with ordinary kitchen tools. After World War I, the US studied its potential use as biological weapon. In collaboration with the British, a ricin bomb was developed and tested, "but apparently never used in battle," according to the "Textbook of Military Medicine."

"It's the third most toxic substance known to mankind," says Bruce Hoffman, an expert on terror at the RAND Corp. in Washington. "But short of injection, it is not very effective, nor is it contagious, as is anthrax."

Still, it's not surprising that it would turn up. "It is the easiest to fabricate," he says. "There are recipes for making it all over the Internet."

There is a recent record of ricin being used in terror attacks - both at home and abroad. As recently as October, a vial of ricin was discovered at the mail facility for the Greenville-Spartanburg International Airport in South Carolina. A letter accompanying the poison complained about new federal regulations requiring more rest for truck drivers, and threatened to taint the local water supply if demands were not met. The FBI has identified the sender as a fleet owner of a tanker company, although no arrests have been made.

Traces of ricin have also been discovered recently at a Paris train station and in a London apartment. Military officials say they found remnants of ricin manufacturing equipment at an Ansar al-Islam camp in northern Iraq during the war campaign. And manuals that described how to manufacture and use the toxin were discovered in Afghanistan, after the US forced out the Taliban regime in 2001.

One of the most well-known uses of it as a killing agent was the alleged assassination of Bulgarian dissident Georgi Markov, who was jabbed with a poisoned umbrella in London in 1978.

But if the anthrax investigation - the largest ever carried out by the FBI - is any indication, it may be some time before a culprit is caught in the latest case.

After 28 months, the FBI still has not solved the anthrax crimes. In tandem with the US Postal Service, it is offering a $2 million reward for information leading to the arrests and convictions of people responsible for mailing the anthrax letters.

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