Federal authorities are investigating suspicious mail sent to the Fort Wayne office of U.S. Senator Jim Banks.

A Banks spokesperson confirmed the mailing to 21Alive News.

The FBI says it is investigating similar letters sent to Senate offices, state election offices and other locations nationwide. Some reportedly contained an unknown substance.

The FBI, U.S. Postal Inspection Service and Capitol Police are working to identify who sent the letters and intercept any additional mailings. Authorities have warned that some letters may still reach their destinations.