Few details have been released in the case, and no arrests have been made. Federal investigators have been searching for the person who sent the letters, which were postmarked Tuesday in Spokane.
The letters were addressed to the downtown post office and the adjacent federal building, but authorities have not released a potential motive. They also have not said whether the letters targeted anyone in particular.
Ricin is a highly toxic substance made from castor beans. As little as 500 micrograms, the size of the head of a pin, can kill an adult if inhaled or ingested.
There have been no reports of illness connected to the letters.
FBI agents, Spokane police and US Postal Service inspectors descended on the three-story apartment building on Saturday morning and the investigation continued into the afternoon.
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