|Sen. Roger Wicker, R-Miss. (AP)|
Milt Leitenberg, a University of Maryland bioterrorism expert, said ricin is a poison derived from the same bean that makes castor oil. According to a Homeland Security Department handbook, ricin is deadliest when inhaled. It is not contagious, but there is no antidote.
The letter to the Mississippi Republican was intercepted at an off-site mail screening facility and never reached the Hill.
Wicker thanked law enforcement officials in a statement for "their hard work and diligence in keeping" those who work in the Capitol safe, adding that the matter is part of an ongoing investigation by Capitol Police and the FBI. "Gayle and I appreciate everyone's thoughts and prayers," he said.
As of Tuesday night, mail delivery had only been stopped to the Senate, not the House.
"It is of concern," Sen. Angus King, I-Maine, said after learning about the incident in a briefing with other senators late Tuesday.
The envelope had a Tennessee postmark and no return address.
The letter inside included an implied threat to effect of: "You haven’t listen to me before. Now you will, even if people have to die," Politico also reported.
Sources say officials are familiar with the person believed to have sent the letter as the person has sent other letters before.
FBI Director Robert Mueller and Homeland Security Secretary Janet Napolitano were on Capitol Hill on Tuesday for a scheduled talk about cyber security. But that briefing morphed into talks about Boston, after the bombings Monday.
Senate Sergeant at Arms Terry Gainer conducted a separate briefing for senators specifically on ricin.
It's unclear whether the letter had any connection to the Boston attack.
The mail-screening system was established after the Anthrax attacks of 2001 that closed the Hart Senate Office Building.
Fox News' Chad Pergram and Mike Levine and The Associated Press contributed to this report.