AUGUSTA — William Smith pleaded guilty Monday to murder and conspiracy to commit murder, for his role in killing Kimberly Mironovas last year in Litchfield.

He was sentenced to 28 years in prison, in a sentence agreed upon by state prosecutors and the defense.

Smith, who is now 17 but who was 15 at the time of the April 22, 2018, murder of 47-year-old Kimberly Mironovas, was one of three teens charged in relation to her death, including Mironovas’ son, Lukas Mironovas.

Police said the three planned to kill Kimberly Mironovas, in part because they were mad at her after she accused them of stealing her marijuana the previous night. Smith, according to police and state prosecutor Katie Sibley, an assistant attorney general, suggested the teens kill Kimberly Mironovas.

Police testifying at his bind over hearing said Smith acknowledged helping strangle Ms. Mironovas, but he said he stopped after a few seconds because he couldn’t go through with it. He said he then backed away, and Lukas Mironovas strangled her and then stabbed her.

Sibley said the state medical examiner determined the cause of Kimberly Mironovas’s death as strangulation and stab wounds, “each of which would have independently resulted in her death.”

Superior Court Justice Michaela Murphy said the 28-year sentence reflected that the teens premeditated the crime with several hours of planning, that Smith didn’t initially take responsibility though he did eventually, the cruelty of the crime and the fact that Ms. Mironovas suffered in pain, as well as Smith’s lack of a prior criminal record and his young age at the time of the crime. She said she read victim impact statements from relatives of Mironovas in considering Smith’s sentence.

Smith declined to speak during his sentencing, other than answering Murphy’s questions.

His attorney, Walter McKee, said while they felt a judge’s decision in July to allow Smith to be tried as an adult was wrong, he acknowledged there would be risks in fighting the case.

“This sentence at least gives Will some hope for the future,” McKee said after Monday’s sentencing. “He will be released in his early 40s and will have a chance at a life for himself. Twenty-eight years is an incredibly long time though, especially for someone who committed the crime when he was just 15 years old. Especially in this case, there are just no winners at all.”

State prosecutor Meg Elam, an assistant attorney general, said the state was gratified Smith “finally agreed to take responsibility for the murder of Kimberly Mironovas.”

“We wish, for the sake of the family, he’d have done it before challenging our attempt to have him held responsible as an adult,” she said. “But even now we’re glad for the family, that they won’t have to endure a trial on Mr. Smith’s role in the murder of Kimberly.”

By pleading guilty, Smith gave up his right to appeal in the case.

Smith could have been sentenced to between 25 years and life in prison.

“It’s difficult to imagine doing such a brutal crime at such a young age,” Elam said outside the courtroom after the sentencing. “I have no idea whether they’ll be able to move past this and become productive citizens or not. But it’s certainly our hope that they will.”

Members of Kimberly Mironovas’ family that were in the courtroom Monday declined, through a court official, to comment.

Smith was indicted by a Kennebec County grand jury in August. A district court judged ruled in July that Smith could be prosecuted as an adult, following a multi-day bind over hearing to determine whether he would be handled in the adult or juvenile justice system.

That meant instead of remaining in the juvenile system, which could have resulted in him spending the next five years at Long Creek Youth Development Center and then being released when he turned 21, he faced the charges of intentional or knowing murder and criminal conspiracy to commit murder as an adult.

Smith is expected to continue to serve his time at Long Creek until he reaches the age of 18, when he’d be transferred to an adult prison.

Smith and Kimberly Mironovas’s son, Lukas Mironovas, 16, were charged with murder and conspiracy to commit murder, while a third teen involved in the incident, Thomas “TJ” Severance, 14, was charged with conspiracy to commit murder.

Severance pleaded guilty to the conspiracy charge and has been committed to Long Creek until he is 21 years old.

Lukas Mironovas waived his right to argue he should be tried as a juvenile as part of a plea agreement with state prosecutors approved by a judge Sept. 18.

In that agreement with the state, Mironovas agreed he would plead guilty to charges of murder and conspiracy to commit murder. The agreement caps his sentence for those crimes at 35 years, meaning he would at least serve the minimum sentence of 25 years, but no more than 35. His sentencing is scheduled for next month.

Lukas Mironovas, of Litchfield, and Smith and Severance, both of Ashland, Massachusetts, were arrested the morning of April 22, 2018, with Smith driving in Mironovas’s car, and taken to the Gardiner Police Department, where state police detectives interviewed them. All three initially stuck to a story Sibley said Smith devised, claiming there had been an intruder into the Mironovas home. But all three ultimately confessed to their roles in the killing.