Initial tests on a letter addressed to President Barack Obama have come up positive for ricin, after an envelope to Senator Roger Wicker of Mississippi was stopped at a mail processing plant in Maryland for traces of the same poison.
Police are in the midst of investigating two different suspicious packages, and have sealed off some parts of two Senate buildings while they conduct tests.
A spokesperson for the authorities said that the third floor of the Senate Russell Building has also been shut down as a result of the investigation. She said the Capitol Police "are investigating a suspicious packaged delivered to our front office."
Ricin, a poison found in castor beans, is highly toxic, and works by getting inside a person's cells and preventing the cells from producing necessary proteins. Without these proteins, they die, according to the Centers for Disease Control and Prevention website.
NBC calls this poison an "assassin's tool," as it is a targeted poison used to attack one person rather than a large group.
University of Maryland bioterrorism expert Milt Leitenberg told NBC, the chemical tests used to gauge a presence of ricin have been incorrect nearly every time, as the initial "rapid detection test," which takes six to eight hours, is not as thorough as the complete test that takes about 48 hours to perform.
"Field and other preliminary tests can produce inconsistent results," the FBI told NBC.
The letter addressed to the White House contained, "a granular substance."
Many are hyper-sensitive to suspicious looking packages or the possibility of terrorist attacks, especially in light of the recent events at the Boston Marathon.
Investigations are still underway. Meanwhile, Senator Carl Levin said he saw a seemingly suspicious letter at his Saginaw, Michigan office, while further equipment is being tested for possibilities of ricin elsewhere.
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