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SPOKANE, Wash. -- The FBI executed a search warrant Saturday in Browne's Addition in relation to threatening letters sent to the Spokane Post Office Tuesday.

The warrant was executed on 1st Avenue. The FBI, US Postal Inspection Service and Spokane Police executed the warrant.

Officials would not say if anyone was taken into custody or had been arrested.

Crews in hazmat suits were on the scene. Police said there is no concern for public safety. The section of 1st Avenue between Oak and Elm is expected to be blocked off for the remainder of Saturday.

Preliminary results showed the two suspicious letters sent to the Downtown Spokane Post Office on Wednesday were laced with ricin. Those tests were confirmed by the US Postal Union, though the FBI would not say if they believe the letters were laced with the poison.

The American Postal Workers Union said one letter was addressed to the Spokane Post Office; the other was addressed to a federal judge in Spokane. Both letters were postmarked May 14.

The organization added there is no reason to believe any employees are at risk from handling the letters. Union leaders said the results show the ricin was not in a form that could be inhaled or ingested.

The Centers for Disease Control and Prevention consider ricin to be one of the deadliest toxins known to man. The poison is found naturally in castor beans and the plant is readily available. Castor beans are typically processed to make castor oil according to the CDC. Ricin can be made from the left over waste material.

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