SELMA, Ala. (AP) - A former state trooper is scheduled to be released from prison this week after serving a 25-year sentence for killing his state trooper girlfriend as she sat in her patrol car near Selma.

State Department of Corrections records show 57-year-old Joe Cecil Duncan finishes his sentence Sunday.

Duncan pleaded guilty to murder in the 1987 shooting death of trooper Elizabeth Cobb. She was shot in the back of her head as she sat behind the wheel. The two had been dating and planning to marry. Cobb had taken out a $350,000 life insurance policy that named Duncan as the beneficiary.

Former Dallas County prosecutor Jim Sullivan told the Montgomery Advertiser ( the killing was cold and calculated. He said Duncan was twice divorced and behind in child-support payments.

Copyright 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.