Spokane: The FBI and other law enforcement agencies executed a search warrant on Saturday in the case of two letters containing the deadly poison ricin that were intercepted this week at a post office in Washington state.
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 on Tuesday. There have been no reports of illness connected to the letters.
"The crude form of the ricin suggests that it does not present a health risk to US Postal Service personnel or to others who may have come in contact with the letter," the agency said in a news release on 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 US senator and a Mississippi judge. A Mississippi man has been arrested in that case.