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 . There is no known antidote . Postal Service tells workers they are n't at risk , union says .
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 .