(CNN) -- Federal authorities are investigating threatening letters allegedly containing deadly ricin in Spokane, Washington, the FBI said Thursday.
The FBI and the U.S. Postal Inspection Service are trying to find the source of the two letters, intercepted Tuesday during a screening procedure at a postal facility in Spokane, FBI spokeswoman Ayn Sandalo Dietrich said.
"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 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.
If inhaled, injected or ingested, less than a pinpoint of ricin can kill a person within 36 to 48 hours because of the failure of the respiratory and circulatory systems. There is no known antidote.