First of all, you may ask, what is Ricin? Well, according to Wikipedia:
Ricin (pron.: /ˈraɪsɪn/), from the castor oil plant Ricinus communis, is a highly toxic, naturally occurring protein. A dose as small as a few grains of table salt can kill an adult human.
1:10 p.m. (EST): A third letter, this one sent to Senator Joe Manchin (D.-W.V.), has been found to contain ricin.
12:45 p.m. (EST): FBI spokesman Paul Bresson has confirmed that the substance found in the letter to the president was ricin.
The Secret Service says that a letter containing a suspicious substance and addressed to President Obama was received on Tuesday at a White House mail facility. This comes just after a letter sent to Senator Roger Wicker (R.-Miss.) was found to contain the poison ricin.