Authorities in hazardous materials suits are searching an apartment in downtown Spokane, Wash., as they investigate the recent discovery of a pair of letters containing the deadly poison ricin.
FBI agents, Spokane police officers and U.S. Postal Inspection Service officials descended on the apartment Saturday morning.
No arrests have been made. An FBI spokesman has not said whether agents are questioning anyone in connection with the case.
Authorities have not released a motive for why the letters were mailed this week.
Despite the hazmat suits, officials say apartment residents are not at risk, and people were seen coming in and out of the building.
Ricin is a highly toxic substance. Tiny amounts can be deadly if inhaled or ingested.
There have been no reports of illness connected to 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.
Two letters containing the substance were intercepted at the downtown Spokane post office Tuesday.
"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 in a news release Thursday.
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 U.S. senator and a Mississippi judge. A Mississippi man has been arrested in that case.
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