Prosecutors announced a plea agreement Thursday with 54-year-old Linda Joyce Lakes of Medical Lake, Wash.
Lakes began serving a prison term at the Women's Correctional Center in Pocatello in 2009 and was released in September.
But while in prison, prosecutors say she mailed at least five letters through the U.S. Postal Service that contained threats to injure and kill an individual. Those letters were addressed to the same residence in Washington, though the name of the intended recipient varied.
Mailing a threatening communication is a felony that carries a prison sentence of up to five years and fines.
Sentencing is scheduled for February.