Ricin is poisonous if inhaled, injected, or ingested and is derived from castor beans.

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NEW: Both letters are intercepted Tuesday during postal screening procedure, FBI says

Preliminary lab tests show ricin on two letters, postal workers union says

FBI and U.S. Postal Inspection Service are investigating source of letters, FBI says

Postal Service tells workers they aren't at risk, union says

CNN  — 

Federal authorities are investigating threatening letters allegedly containing deadly ricin in Spokane, Washington, the FBI said Thursday.

The FBI said Tuesday the U.S. Postal Inspection Service have been trying to find the source of two letters intercepted during a screening procedure at the postal facility.

“While we cannot offer further comment on this incident, we stress that law enforcement agencies will continue to assess and address the full spectrum of potential threats,” she said.

The American Postal Workers Union was notified of the two suspicious letters by the Postal Service, the union said.

One letter was addressed to the Spokane Post Office and the other to a federal judge in Spokane, the union said. Both were postmarked May 14, the union said.

Postal Service management told employees they weren’t at risk from handling the letters because the suspected substance was not in a form that could be inhaled or readily ingested, the union said. Preliminary lab tests showed the substance to be ricin, the union said.

The union is monitoring the situation, President Cliff Guffey said.

“Our members’ safety is our primary concern,” he said in a statement.

In an apparently unrelated case, ricin-tainted letters sent to President Barack Obama and others touched off high anxieties in Washington and elsewhere last month.

James Everett Dutschke, 41, of Tupelo, Mississippi, has been charged with possession and use of a biological agent as a weapon in connection with those letters sent to Obama and others.

Ricin is derived from castor beans. If inhaled, injected or ingested, less than a pinpoint of the toxin can kill a person within 36 to 48 hours because of the failure of the respiratory and circulatory systems. There is no known antidote.