A letter addressed to President Obama has initially tested positive for the poisonous substance ricin, according to White House officials.
The White House held a press conference to address the situation.
On Tuesday a similar letter — both are psotmarked from Memphis, Tenn. on April 8 — was sent to an office of Sen. Roger Wicker (R-Miss.) tested positive for the deadly poison.
A bomb squad is also working at two Senate buildings because of suspicious packages.
Ricin is a poison naturally found in castor beans that prevents the a person's cells from making any proteins. It can come in the forms of a powder, a mist, or a pellet, or it can be dissolved in water or weak acid.
As little as 500 micrograms — smaller than a grain of sand — can kill an adult.
Mail to the Senate has been halted as of yesterday. Last night Sen. Claire McCaskill (D-Mo.) told CNN that police have already identified a suspect who sends a lot of letters to lawmakers.
More from Levine:
Both letters signed, "I am KC & I approve this message." As of yesterday, #FBI "pursuing multiple leads on potential senders," bulletin says
— Mike Levine (@MikeLevineFNC) April 17, 2013
An unnamed investigator toldNBC News that there are several scenarios in which the tests result in a false positive. These include that the highly sensitive tests could have picked up non-toxic byproducts of the castor bean plant as opposed to ricin.
"Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin," the bureau told CNN. "Those tests are in the process of being conducted and generally take from 24 to 48 hours."
In 2004 three Senate offices were shut down because of a ricin scare. Postal workers began sorting through mail at an off-site facility after anthrax attacks in 2001 targeted lawmakers including then-Majority Leader Tom Daschle and Vermont Sen. Patrick Leahy.
The letter sent to Wicker, which tested positive three times in the field, has been sent to a Maryland lab for further testing.
SEE ALSO: Envelope Sent To US Senator Tests Positive For Deadly Poison
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