NEW : Both letters are intercepted Tuesday during postal inspection procedure , FBI says . Preliminary lab results show ricin on both letters , postal workers union says . FBI and U.S . Postal Inspection Service are investigating source of risk , union says . The FBI and the U.S . Postal Service tells workers they are n't at risk , union says .
NEW : Both letters are intercepted Tuesday during postal inspection procedure , FBI says . Preliminary lab results show ricin on both letters , postal workers union says . FBI and U.S . Postal Inspection Service are investigating source of risk , union says . Postal Service tells workers they are n't at risk , union says .