Police say the investigation has focused on a neighbourhood near downtown Spokane.
The FBI, US Postal Inspection Service and Spokane police are involved, but further details were not immediately available.
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.
Two letters containing the substance were intercepted at the downtown Spokane post office Tuesday. There have been no reports of illness connected to the letters.
The Postal Service said it has received no other reports of similar letters. However, the agency did investigate a suspicious package sent to a federal judge in Spokane this week and found there was no hazard.
The Spokane investigation comes a month after letters containing ricin were addressed to President Barack Obama, a US senator and a Mississippi judge. A Mississippi man has been arrested in that case.
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