Manna also pleaded guilty Thursday to attempted first-degree murder and a handgun violation and waived his appeal rights. In return, Frederick County State's Attorney Scott Rolle dropped an assault charge and a second handgun charge and agreed to the parole provision.
Prosecutors had planned to seek life without parole if Manna were convicted at a trial that was to start Thursday.
''The bottom line is this: we didn't give up anything; he gave up a lot,'' Rolle said. ''Our view is, he will serve the rest of his life in prison.''
Manna killed Linda Delauter, 53, of Myersville with a shot to the chest when she answered his knock on her front door on the night of Jan. 6, 1997.
Minutes earlier, he had fired three shots through the front door of a home in nearby Wolfsville, narrowly missing Linda Grossnickle, 45.
According to court documents, Manna told police he wanted to hurt the women and Mrs. Delauter's husband, Charles, because they had supported his ex-wife, Barbara, in her decision to leave him in August 1996.
But it was the breakup of Manna's subsequent relationship with Candace Sanchez, of Waynesboro, Pa., that triggered the violence, according to a statement of facts read by Assistant State's Attorney David Callahan. After briefly moving in with Manna, Sanchez told him on the night of the shootings she was returning to her husband, Richard, Callahan said.
Manna is accused in Pennsylvania of breaking into the Sanchez home and shooting Richard Sanchez several times.
Manna, who ran a lime-spreading business, told police he drank at least a quart of tequila that night, Callahan said.
Charles Delauter said he agreed to the plea bargain because he couldn't be certain Manna would be convicted at trial.
''I couldn't chance that. This way, I've got him away for 15 years without the ability to appeal. Hopefully, he'll spend the rest of his life in jail,'' Delauter said.
Manna spoke in court Thursday only to answer questions from Circuit Judge John H. Tisdale and Public Defender Franklin Stillrich about the terms of the plea bargain.
When Stillrich asked, ''Are you in fact guilty of these three charges,'' Manna replied, ''Yes.''