William Smith, 17, (right) was one of three teenagers charged in the 2018 killing of Kimberly Mironovas in Litchfield. Credit: CBS 13 | WGME

AUGUSTA, Maine — A Massachusetts teenager charged in the killing of a 47-year-old Maine woman was sentenced to 28 years in prison on Monday.

William Smith, 17, was one of three teenagers charged in the 2018 killing of Kimberly Mironovas in Litchfield.

Smith pleaded guilty to intentional or knowing murder and criminal conspiracy to commit murder. He accepted a plea deal for a 28-year sentence.

Judge Andrew Benson found that Smith was “the prime mover” in the plan.

Kimberly Mironovas’ 16-year-old son, Lukas Mironovas, pleaded guilty to murder in the death of his mother. He is due to be sentenced in December.

Another Massachusetts teenager, who was 13 at the time of the killing, pleaded guilty to conspiring to kill Kimberly Mironovas. He will remain in juvenile detention until he is 21.

Court documents indicate that before Mironovas was killed, she confronted the teens about missing marijuana and refused to drive Smith and the then-13-year-old back to Massachusetts.

Prosecutors have said Lukas Mironovas and the two friends planned to spike his mother’s drink with crushed prescription pills. When the plan failed, they strangled and stabbed her.

The Associated Press contributed to this report.