AUGUSTA -A Fayette teenager was sentenced to 25 years in prison, with nine of them suspended, after he pleaded guilty Friday to an adult manslaughter charge for beating Marlee Johnston, 14, of Fayette, to death with a bat last fall.

Patrick Armstrong, 15, will serve the 16 years after he spends up to the age of 21 in the Mountain View Youth Development Center in Charleston after he pleaded guilty to two felony counts of burglary Friday in the same Lovejoy Pond neighborhood in which he and Johnston lived.

Justice Charles LaVerdiere, presiding in Augusta district Court, agreed to try Armstrong as an adult and to the negotiated plea agreement that called for the state to drop a murder charge in an exchange for the guilty plea. He also ordered Armstrong to serve four years’ probation and to pay restitution of nearly $5,000 in funeral costs.

Armstrong, 14 at the time of Johnston’s death, admitted to state testimony that he struck Johnston in the head repeatedly on Nov. 26, 2005, and then dragged her into Lovejoy Pond.

Johnston’s older brother, Alec Johnston, pulled his sister from the water before he went to the Armstrong’s house for help, Maine State Police Detective Adam Kelley said, answering state prosecutor’s questions.

Betty Armstrong, a registered nurse, told her son to call 911 before she went to give cardiopulmonary resuscitation on Marlee Johnston, who had stopped by the Armstrong’s house earlier that day to ask Patrick to go for a walk, Kelley said. A short time later Patrick returned and told his mother that Johnston’s dog had a sore paw and they parted ways, Kelley said.

An autopsy showed that Johnston died as a result of blunt-force trauma to the head, Kelley said. The state medical examiner found multiple lacerations inflicted in six to eight blows “severe enough to fracture her skull and “ultimately caused her death, Kelley said.

There was no evidence of sexual assault, he said.

Armstrong gave conflicting accounts and denied killing Johnston, Kelley said.

State forensic analyst Catharine MacMillan testified DNA found on the bat, Armstrong’s pants and Johnston’s hat matched both Armstrong and Johnston.

She also said Armstrong could not be excluded as being the donor of light traces of sperm found on the outside of the hat and his pants.

Dr. Kathryn Thomas, a forensic psychologist, said Armstrong’s versions of the events changed several times while he was in state custody.

She described him, on the outside, as bright, articulate, having good eye contact and presenting very well even under adverse conditions. But underneath, she said, he was narcissistic, antisocial and able to lie straight-faced. He also had characteristics of someone wanting to attract attention, of being a risk-taker and someone willing to violate the rights of others to get what he wanted, Thomas said.

Armstrong showed no remorse for what happened, she said, instead he was more concerned about the impact on himself and his family than he was for the victim and her family.

He told her he got more and more upset with Johnston as she talked about how well her life was going as they walked because his life wasn’t going well, she said.

He said he was “getting her mixed up with a former girlfriend ” in his mind, Thomas said.

Armstrong showed no emotion during the state’s testimony, but when Johnston’s parents, Ted Johnston and Marlene Thibodeau, spoke, his eyes teared.

“The pain is not something that will ever go away. There is not a moment that goes by that I don’t hurt,” said Johnston, who was visibly shaking at times during the hearing.

There will never be more plans to make, morning hugs, girl talk or shopping together, said Thibodeau, who was crying.

“I love her with all my heart. From the depths of my soul, I love her and I’ll miss her forever,” she said.

“You have committed the most despicable act against a human being” LaVerdiere said to Armstrong. “You have taken the life of an innocent young woman, and you did so selfishly.”

Armstrong changed the lives of all involved, the justice said.

Confinement is first punishment for the terrible act, he said, “but it is also an opportunity to find a way to rehabilitate yourself, to find a way to … change.”

He went on to tell Armstrong that his time in confinement should be used to make himself better, and to find a way to make up for what he’s done.

Attorney General Steven Rowe, who accompanied Deputy Attorney General Bill Stokes and Assistant Attorney General Andrew Benson to the courthouse, said outside that, “It was an unusual sentence, but the circumstances of this crime are unusual. Our hearts go out to the Johnston family.”

Both Benson and Armstrong’s lawyer, Walt McKee, said both sides had risks associated with the case if it went to trial.

The state dropped the murder charge to go with the plea agreement on manslaughter in the interest of public safety, Benson said.

McKee said the unique sentence, unprecedented in Maine, of combining a juvenile sentence and adult sentence was thinking outside the legal box.

“I think the result took into account significant risks on both sides,” McKee said.