An envelope meant to be opened by a United States Senator is undergoing further tests after it was found to contain ricin.

The envelope was intercepted at the U.S. Capitol's off-site mail facility in Washington DC, after allegedly failing a routine mail test. Two more tests were performed and both came back positive.

CNN reports the envelope was then sent to a Maryland lab for more tests.

According to the CDC, ricin is a highly toxic protein formed as a waste product when making castor oil. When ingested, it can cause severe vomiting, tightness in the chest and pulmonary edema. If left untreated, ricin poisoning can be fatal.

11:15AM 4/17

Reports are surfacing that a second letter has tested positive for ricin.  There are conflicting reports as to the addressee.  The letter was either sent to another US senator or to President Barack Obama.  Either way, the letter was detected at a postal sorting facility.

It's worth remembering that following the 9/11 attacks, there was a rash of anthrax letters unrelated to the terrorist cell that carried out the hijackings.

12:30PM

A suspicious package arrived at the Saginaw office of Michigan Senator Carl Levin

9:31 p.m. - Authorities have arrested Paul Kevin Curtis from Tupelo, Mississippi for sending the envelopes tainted with ricin.