St. JOHNSBURY, Vt. (AP) — A Vermont couple accused of luring a popular St. Johnsbury teacher from her home, killing her and dumping her body in the Connecticut River in March have pleaded not guilty to upgraded murder charges.

Thirty-year-old Allen and 33-year-old Patricia Prue of Waterford appeared separately Tuesday in Vermont Superior Court in St. Johnsbury.

Allen Prue is now charged with first-degree murder; Patricia Prue is now charged with aggravated murder in the death of Melissa Jenkins.

Melissa Jenkins (Photo Credit: Vermont State Police)

Prosecutors say the Prues killed Jenkins while sexually assaulting her.

A police affidavit say the Prues planned in advance to kill the 33-year-old single mother, buying a stun gun and prepaid cellphone before the crime, and then strangled, beat and electrocuted her.

The Prues previously pleaded not guilty to second-degree murder charges.

Copyright 2012 The Associated Press.