Authorities say envelopes that may contain the naturally occurring poison ricin were addressed to President Barack Obama and to Sen. Roger Wicker, (R-Mississippi) Tuesday.
The act shook Washington only one day after bombings during the Boston Marathon claimed three lives and injured more than 100 people.
Ricin is a highly toxic substance derived from castor beans. As little as 500 micrograms or a few grains can kill an adult when inhaled, ingested or injected. There is no specific test for exposure and no antidote once exposed. It is easy to produce and cheap to obtain, making it a popular choice to attack individuals. Ricin can be injected, used to poison food or spread in the air when made into a powder.
According to the CDC, the symptoms of ricin poisoning are difficulty breathing, fever, cough, nausea and tightness in the chest. Heavy sweating may follow, as well as fluid building up in the lungs. These symptoms can show up as soon as a few hours after the poisoning and can kill an individual within three to five days.
However, experts say ricin is likely not a weapon of mass destruction. Only one confirmed death from ricin has been documented so far. In 1978, Bulgarian Georgi Markov was assassinated when he was jabbed with ricin. Markov died four days later.
A previous ricin scare hit the Capitol in 2004, when an envelope containing the substance was found in the mailroom of Sen. Bill Frist's office. However, no one reported ill effects from the substance.
"The FBI has the lead in that investigation, of course, and has said in its statement [that] they will conducting further tests to determine what the nature of the substance is. Of course, there was another [letter] that was intercepted by Capitol Police that was sent to a United States senator and that also is subject to an investigation by the FBI. The president has, of course, been briefed on these letters last night and again this morning," White House Spokesman Jay Carney said.
