Ricin is a highly toxic substance derived from castor beans. As little as 500 micrograms -- an amount the size of the head of a pin -- can kill an adult. There is no specific test for exposure and no antidote once exposed.
http://www.cnn.com/2013/04/17/politics/tainted-letter-intercepted/index.html?hpt=hp_t1
A suspicious substance has been found in letters mailed to Obama and Sen. Wicker. It is believed to be Ricin but testing can take 24-48 hours. They say they have nothing to connect this to the Boston marathon attack but if you ask me there is no such thing as a coincidence.
In my opinion, if this was an Islamist group they would have claimed it. If it was a domestic group it would have been a small one event attack. The fact that this is now likely multi layered suggests it is a bigger attack.
Who do think is doing this? NK?