( 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 can not 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 were n'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 .
NEW : Both letters are intercepted Tuesday during postal screening procedure , FBI says . Comment: FBI and U.S . Postal Inspection Service are investigating source of letters , FBI says . Postal Service tells workers they are n't at risk , union says .