A teenager who beat and strangled a teacher to death at a state-run juvenile detention center in Prince George’s County was sentenced to 85 years in prison Friday as part of a plea agreement with prosecutors.

Brian Wonsom, now 15, pleaded guilty during the hearing to first-degree murder and attempted first-degree rape in the February 2010 attack at the Cheltenham Youth Facility and to attempted first-degree murder in a separate, 2009 case. Although Wonsom appeared to be writing something at one point during the proceedings, he declined Prince George’s County Circuit Judge Toni E. Clarke’s offer to let him comment with a simple no.

For such a brutal case involving such a young defendant, the conclusion was somewhat subdued. None of Wonsom’s relatives went to court, neither did any relatives of the victim, Hannah Wheeling, 65. Wonsom, who has grown a light goatee and mustache, showed almost no emotion.

Wonsom was 13 and in the Cheltenham Youth Facility on burglary charges when he became a suspect in Wheeling’s killing. While taking a test with Wheeling on Feb. 17, Wonsom apparently beat and strangled the youth facility teacher, then sexually assaulted her, prosecutors said Friday. Another staff member found her body the next day, prosecutors said.

Wonsom’s bloody sweat shirt was found in a stairwell at the facility, near Wheeling’s body, prosecutors said. Investigators eventually determined that semen found on Wheeling belonged to the boy, and they determined Wheeling was sexually assaulted after she had been killed, prosecutors said.

Wonsom was held after the attack, and a judge eventually ruled that he should be tried as an adult. He was indicted in March.

After handing down the 85-year sentence agreed upon by prosecutors and Wonsom’s attorney, Clarke told Wonsom he was “not fit to live in society.” He will spend at least 42.5 years behind bars as part of his plea agreement, as he will be eligible for parole after 50 percent of his sentence is served, according to Clarke and Allen Wolf, Wonsom’s attorney.

In addition to admitting to the Cheltenham killing, Wonsom pleaded guilty to a separate, 2009 incident in which he was accused of stabbing a neighbor in Laurel. That case, prosecutors said, emerged after Wonsom was a suspect in the Cheltenham murder and investigators were able to compare his fingerprints to those from the 2009 incident.

After the hearing, prosecutors said they were satisfied that Wonsom would be taken off the streets, even if not for his entire life. They noted that psychiatric experts had said previously the teenager had “sociopathic” tendencies that would require decades of intensive treatment if he had any chance for recovery.

“It’s a tremendous tragedy,” Prince George’s State’s Attorney Angela Alsobrooks said. “He was a menace.”