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.
Federal law enforcement officers are working to intercept the letters, according to the notice.
This is a developing story. Please check back for updates.