White House officials have confirmed that a letter addressed to President Obama at the White House does contain traces of the toxin ricin. It's also confirmed that it's from the same sender who mailed a similarly spiked letter to a senator.
According to an editor at Reuters (via Twitter), the official position is that there's no apparent link between the ricin events and the recent terrorist bombings in Boston. Separately CNN on Twitter noted that the Hart Senate office building was being evacuated because of a suspicious package which had been delivered to the front office.
Ricin is a toxin formed as part of the by-products of the process that turns castor beans into castor oil. It causes cell death, and is very powerful--a single molecule can kill a cell. But administering ricin in a way that can kill a person is not straightforward. Famously it was used to assassinate Bulgarian dissident writer Georgi Markov in London in 1978: Markov was stabbed by an umbrella that administered the toxin.
Update: Reuters reports that the Hart office is open again because tests on the package were negative.
[Image: By Flickr user harshlight]