A prison inmate accused of sending threatening letters to Gov. Peter Shumlin has pleaded guilty to three federal charges related to the threats.

Dakota Gardner, 22, changed his plea from innocent to guilty on the three charges Tuesday in U.S. District Court in Rutland.

Gardner, who was found mentally competent to stand trial last week, changed his plea after accepting a deal from the U.S. attorney’s office. Prosecutors agreed to drop a fourth charge against him and to grant him sentencing credits for accepting responsibility for the crimes in a timely fashion.

Gardner, who could serve a maximum of 15 years behind bars on the remaining charges, is scheduled to be sentenced on July 15.

Last year, Vermont State Police were notified of a threatening letter that had been sent to Shumlin’s office from an inmate at the Springfield prison.

Police said Gardner threatened violence against Shumlin and his family while he was in prison after a 2009 arrest in Brattleboro for aggravated assault.