NBC News: Ricin Tests May Have Been Wrong:

"There is a new theory emerging about the ricin scare two weeks ago.

NBC News has learned investigators are looking into the possibility that there never was any ricin attack in the first place...

Since ricin comes from the castor bean, and some nontoxic parts of the plant are used to make paper, it might be possible that the tests found traces of the plant, but not ricin."