Preliminary lab tests showed the substance to be ricin , the union said . NEW : Both letters are intercepted Tuesday during postal screening procedure , FBI says . Preliminary lab tests show ricin on One letters , Postal Service says . FBI and Spokane . FBI are investigating source of letters , FBI says . Postal Inspection Service tells workers they are n't at risk , union says .
NEW : Both letters are intercepted Tuesday during postal screening procedure , FBI says . Preliminary lab tests show ricin on One letters , Postal Service says . FBI and Spokane . FBI are investigating source of letters , FBI says . Postal Inspection Service tells workers they are n't at risk , union says .