The FBI said there have been no reports of illness connected to the letters, which were intercepted Wednesday.
The FBI, along with Postal Service investigators, were searching for the sender of the letters, which were postmarked Tuesday in Spokane.
"We're aware of threatening letters intercepted Wednesday during a screening procedure at a U.S. Postal Service facility in Spokane," said FBI spokeswoman Ayn Sandalo Dietrich. "The FBI is working with law enforcement partners ... to investigate the source of the letters."
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.
The Postal Service said one of the letters was addressed to the Spokane post office.
"The letter included an articulated bomb threat and material that tested positive for the presence of ricin," the Postal Service said in a news release Thursday.
"The crude form of the ricin suggests that it does not present a health risk to U.S. Postal Service personnel or to others who may have come in contact with the letter," the agency said.
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.
This incident comes a month after letters containing ricin were addressed to President Barack Obama, a U.S. senator and a Mississippi judge. A Mississippi man has been arrested in that incident.
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