COLUMBUS (AP) — An Ohio inmate has pleaded guilty in federal court to mailing more than a dozen threatening letters he claimed contained anthrax or fentanyl but proved to be harmless powder.

The U.S. Attorney’s Office in Columbus said 36-year-old Sean Heisa offered his plea Wednesday to charges of making false information or hoaxes and mailing threatening communications.

Prosecutors say the letters were sent between July 2017 and July 2018 to city, court and prison officials, to two Ohio newspapers and to then Ohio Attorney General Mike DeWine, who’s now the governor. Some of letters threatened the use of explosive devices.

Heisa is incarcerated at the Southern Ohio Correctional Institution in Lucasville serving sentences for numerous convictions from five different counties.

A public defender could not be reached late Wednesday.