A 16-year-old boy pleaded guilty Tuesday to being one of three teens who participated in the ambush robbery and killing of a grandmother of one of the defendants in her home on the eastern edge of Willmar.

Devon Jenkins, of Willmar, admitted in Kandiyohi County District Court to unintentional second-degree murder in the death of Lila Warwick, 79, on July 29.

In exchange for his plea, Jenkins will receive an extended juvenile sentence until he’s 21, at which point a 15-year adult prison sentence stayed with conditions would take effect.

He was accused of driving the accused killer, Brok Junkermeier, 19, to the home and remaining in the car as a lookout that morning.

Junkermeier and Robert Warwick, 17, await trial, each charged in adult court with first-degree murder in the death of Warwick’s grandmother. Conviction for the two could be life in prison without parole.

Prosecutors allege that Robert Warwick hatched the plan that led to Junkermeier’s attack on Lila Warwick in her home, giving the other teens information about his grandmother’s house and habits.

Junkermeier allegedly slashed Lila Warwick’s hand with a swordlike knife and forced her to write him a check for $1,500 before choking and stabbing her to death, according to charges filed in August.

Lila Warwick’s daughter, Cheri Ekbom, told investigators that nephew Robert Warwick was abusing drugs and that his grandmother had been trying to get him help, according to the charges.

Jenkins will remain in the Prairie Lakes Youth Detention Center in Willmar until his sentencing in January.