An envelope intercepted Tuesday afternoon, which tested positive for the poison ricin, was sent to Mississippi Republican Senator Roger Wicker.
The letter had a Memphis, Tenn., postmark and no return address.
Since anthrax-tainted letters were mailed to Capitol Hill in 2001, mail is screened at an off-site facility. The ricin-laced letter never reached the Hill. Inside the enveleope a letter read: "You haven’t listen to me before. Now you will, even if people have to die.”
Sen. Claire McCaskill, D-Mo., told reporters that authorities already identified a suspect in the crime, but authorities declined to comment.
"The person that is a suspect writes a lot of letters to members," McCaskill said after she attended a classified briefing with other Senate members.
"The bottom line is, the process we have in place worked," she noted.
Ricin is a poison found naturally in castor beans. Inside the body, it kills cells by preventing them from making proteins, according to the Centers for Disease Control and Prevention. There is currently no antidote for ricin poisoning. It can enter the body through the inhalation, ingestion, or contact with the skin or eyes.
Senate Sergeant-at-Arms Terrance Gainer said the FBI will perform additional testing on the letter. "Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin," according to the bureau. "Those tests are in the process of being conducted and generally take from 24 to 48 hours."
It is still unclear whether the ricin letter had anything to do with the Boston Marathon attack.
Gainer briefed senators Tuesday specifically on ricin.
The anthrax-laced letters sent to the media and Capitol Hill in 2001, shortly after the 9/11 attacks, killed five people and infected 17 others. The letters were eventually traced to Bruce Edward Evans, an American microbiologist and senior biodefense researcher of the U.S. Army Medical Research Institute of Infectious Diseases.