A suspicious envelope containing ricin or another poison was intercepted at an off-site mail facility that was headed to Sen. Roger Wicker, R-MS, according to Senate Majority Leader Harry Reid, D-NV.
"It was caught in the screening facility. That's why we have an off-site screening facility for mail," said Sen. Claire McCaskill, D-MO, to CNN.
The envelope tested positive in a routine test, and in two other subsequent tests, law enforcement officials tell CNN. The mail facility is closed for more testing.
McCaskill told Politico the letter came from a person who writes to lawmakers, but declined to identify the individual.
The Associated Press reports lawmakers were informed of the incident in a closed-door meeting about the Boston Marathon bombings.
CNN reports Wicker now has a protective detail.
Ricin is extremely poisonous and is made from castor beans. According the New York Department of Health, accidental exposure to ricin is "highly unlikely."
No antidote exists for ricin poisoning and death could take from 36 to 72 hours after exposure. Ricin can be in the form powder, a mist or a pellet and can be dissolved in liquid.
Symptoms, which can present 4 to 6 hours after exposure, include respiratory distress (difficulty breathing), fever, cough, nausea, and tightness in the chest. Heavy sweating may follow as well as fluid building up in the lungs," according to the Center for Disease Control and Prevention.
The incident is reminiscent of the anthrax scare after September 11 where five people died and 17 others were exposed. Envelopes were sent to congress and NBC News containing anthrax.
How mail was processed and sent to congress was changed as a result.
Ricin has been used in other countries in assassinations, such as the poisoning of Georgi Markov, a Bulgarian dissident, in 1978. He was jabbed by the tip of an umbrella in London and died four days later.