GREENVILLE, S.C. (AP) - Investigators have sent out hundreds of thousands of fliers to Upstate residents looking for clues to who is responsible for a vial of the deadly poison ricin found at a post office here in October.

The FBI is offering a $100,000 reward in the case. The fliers were sent to homes in the 296 and 293 ZIP codes.

Investigators don't know where the person who signed the letter "Fallen Angel" is from, but they think he or she is familiar with the area because the post office where the package marked "CAUTION RICIN poison" was found isn't used by the general public. The facility serves the Greenville-Spartanburg International Airport.

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.

The letter included with the package threatened to "start dumping" ricin if new federal trucking regulations requiring drivers to rest longer weren't repealed.

Information from: The Greenville News