Letters that threatened federal offices around Kentucky contained a powdered artificial sweetener and are believed to have been sent by a prison inmate.

The Lexington Herald-Leader quoted U.S. Marshal Mike Klein saying he understood between 25 and 50 of the letters were mailed by an inmate at the Big Sandy federal prison in Martin County.

The letters stated the powder in them contained anthrax. Tests showed there were no harmful substances.

The newspaper reported no spokesman was available at the prison to confirm information about the inmate, including his name.

Letters were received Monday at federal courthouses in London, Frankfort and Pikeville. They also were sent to the offices of U.S. Reps. Brett Guthrie, John Yarmuth, Geoff Davis and Ed Whitfield.