Feds investigating letters with 'suspicious substances' sent to congressional offices

Federal law enforcement officers said they are working to intercept the letters.
The FBI, U.S. Capitol Police and U.S. Postal Inspection Service are investigating "multiple letters containing suspicious substances that were mailed to various addresses nationwide, including multiple Congressional offices," according to a notice sent to Senate staff on Thursday and described to ABC News.
Preliminary information suggests there are over 100 letters circulating in the U.S. mail system that apparently contain white powder, law enforcement sources told ABC News.
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