A letter intended for Sen. Roger Wicker (R-Miss.) was intercepted by authorities after it tested positive for the poison ricin, HuffPost reported on Tuesday.
UPDATE (April 17): Subsequent reports on Wednesday reveal that authorities intercepted two additional letters laced with ricin, including one addressed to President Barack Obama.
But what is ricin and what makes it so dangerous?
Ricin is a naturally occurring compound found in castor beans that is released when the beans are damaged -- such as if they are chewed or processed. The toxic substance is a byproduct in the manufacturing of castor oil -- a commonly used laxative. There is no risk of ricin poisoning with castor oil because the toxicant is water-soluble and doesn't press into the oil.
Just one milligram of ricin can kill an adult, according to a Cornell University fact sheet.
Ricin is a cytotoxin that inhibits the synthesis of proteins that are important for cellular health. Without those proteins -- specifically eukaryotic ribosomes -- cells die, according to a Centers for Disease Control and Prevention report.
The CDC reports that symptoms of ricin poisoning vary based on how it was ingested (it is poisonous whether it is inhaled, ingested or from skin or eye exposure), but some symptoms include: difficulty breathing, fever, cough, nausea, and tightness in the chest for inhalation; nausea and diarrhea for ingestion; and redness and pain for contact. One can die from the exposure within 36 to 72 hours, they said.
There is no antidote for ricin and so the best treatment is to rid the body as quickly as possible of the poison, according to the CDC.