GREENVILLE, S.C. - Truckers nationwide are on the lookout for the person who delivered a package containing the deadly poison ricin to a Greenville mail center last month.

The American Trucking Association told thousands of truckers in its safety network to watch for individuals acting suspiciously, especially people displaying any kind of "attack planning characteristics."

Truckers were asked to call the association's Highway Watch program to pass information to the FBI and South Carolina's Joint Terrorism Task Force, which are investigating the Oct. 15 incident.

The Highway Watch network involves thousands of truckers who are trained to spot unsafe or suspicious activity. The network receives between 300 and 500 calls every month, reporting everything from accidents to spills to normal unsafe operation issues, said Jack Legler, director of security for the Trucking Association.

"Mixed in with all of that is the observation of unusual activities, which can either be criminal or possibly terrorist in nature," he said.

A letter in the envelope contained a warning that large amounts of ricin would be dumped into drinking reservoirs around the country if the federal government did not change a rule requiring truckers to rest after 10 hours on the road

The letter threatened the Federal Motor Carrier Safety Administration, which regulates trucking. The letter writer threatened to use "large quantities" of ricin.

Ricin is derived from the castor bean plant, is relatively easy to make and can be deadly in very small doses. 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.

Spokesmen for the FBI have declined to answer specific questions about the investigation because it is ongoing.

Law enforcement authorities were notified six hours after the package was found at Greenville's airport mail center, and authorities did not inform the public, postal workers or other emergency services about the poison until it was tested and positively identified a week later.

The poison was in a water-tight, metal vial. No one has been reported harmed in the incident.

The FBI has polygraphed postal workers and at least one truck driver, according to the president of a local chapter of the postal workers union.