A letter laced with the deadly poison ricin was sent to the office of Sen. Roger Wicker, R-Miss., but the envelope was intercepted at a mail facility outside Washington, according to officials.
"Postal officials and law enforcement did an excellent job in detecting and preventing this threat before it reached the Capitol," said Don Stewart, spokesman for Senate Minority Leader Mitch McConnell, R-Ky. "The protective measures worked."
Senators learned of the poisoned letter at a closed-door briefing Tuesday evening with Homeland Security Secretary Janet Napolitano and FBI Director Robert Mueller. The two agency heads were at the Capitol to brief lawmakers about Monday's bombing in Boston.
The letter was tested three times, with each test confirming the presence of the potentially deadly substance, sources said.
Mail to congressional offices was suspended after the letter was found.
"This matter is part of an ongoing investigation by the United States Capitol Police and FBI," said Wicker. "I want to thank our law enforcement officials for their hard work and diligence in keeping those of us who work in the Capitol complex safe. Gayle and I appreciate everyone's thoughts and prayers."
According to the Centers for Disease Control and Prevention, ricin is a poison derived from castor beans and can poison people through food, air or water.
"It would take a deliberate act to make ricin and use it to poison people," the CDC states on its website. "Unintentional exposure to ricin is highly unlikely, except through the ingestion of castor beans."
Ricin was detected in mail sent to the Senate in 2004, prompting the closure of several Senate offices. Later, however, investigators found the positive reading for ricin may have been a false one triggered by a byproduct of the castor bean plant that may have been used in producing the paper.
A ricin-laced letter was mailed to the White House in the fall of 2003 and intercepted at a processing facility. The letter was sent by someone who assumed the pseudonym "Fallen Angel." The sender expressed anger over changes in federal trucking regulations. Despite a hefty reward offer, the sender was never caught.
In 2001, anthrax-laden letters killed five people across the country and infected many others, including dozens of Capitol Hill staffers who opened spore-laden letters sent to several senators.
The anthrax attack prompted Capitol officials to alter the mail service, requiring that all letters be tested off-site and most of them scanned rather than personally delivered to offices.