A letter inside the package said the author could make much more ricin and will "start dumping" large quantities of the poison if new federal trucking rules went in effect, according to information released by the FBI and other federal agencies Thursday.
The rules, which require more rest hours for truck drivers, took effect Sunday.
The letter, signed "Fallen Angel," said the author was "a fleet owner of a tanker company."
No suspects have been arrested and no illnesses have been reported since the package was found Oct. 15 at the postal facility serving Greenville-Spartanburg International Airport. The postal center was temporarily closed while health officials tested it. No signs of contamination were found.
The FBI and other federal agencies set up a toll-free number, (866) 839-6241, for information about the incident.
The FBI has yet to make an arrest or identify a suspect in a similar investigation, the probe of the 2001 anthrax letters that killed five people and sickened 17 others..
Ricin, derived from the castor bean plant, can be deadly. When inhaled or ingested, fever, cough, shortness of breath, chest tightness and low blood pressure can occur within eight hours. Death can come between 36 and 72 hours after exposure. There is no antidote.
The FBI has previously warned that some chemical and biological weapons could be improvised by terrorists using readily available materials. Examples include ricin.
Last year, traces of the deadly poison ricin were found in a London apartment. Reports have indicated a suspicion that the ricin was going to be used to poison food at a nearby military installation.
In 1978, ricin was used by Bulgarian intelligence agents in London to assassinate dissident Georgi Markov, whom they pricked with a poisoned umbrella tip.