ALLEGAN COUNTY, MI - Joshua Keyzer, who was 15 when he raped and killed his cousin, pleaded guilty Monday, May, 11 to felony murder.

In a halting voice, Keyzer admitted he killed Kassandra Keyzer, 21, while trying to rape her.

"I, uh, killed her, ma'am," he told Allegan County Circuit Judge Margaret Zuzich-Bakker.

He also pleaded guilty to attempted first-degree criminal sexual conduct.

Keyzer sat next to his attorney, James Mikel McEwen. They were shown on a video screen via a link at the Allegan County Juvenile Detention Center.

Many family members attended the hearing but left with police through a door that generally stays locked at the courthouse. Some wept.

As part of a plea agreement, Keyzer will not face life in prison without hope for parole, which is the mandatory sentence for adults convicted of first-degree or felony murder. Prosecutors will seek a sentence of 40 to 60 years in prison when Keyzer, now 16, is sentenced on June 15.

Keyzer was living part-time at his grandmother's home in the 3000-block of 10th Avenue in Wayland Township when the killing occurred on June 21.

The grandmother called 911 after being slashed in the neck.

Police found Keyzer covered in blood and holding a knife on the roof of the house. He fell from the roof onto bushes and tried to run but was quickly subdued by state police with a Taser.

The scene inside told a horror story.

Kassandra Keyzer, the mother of a 2-year-old boy and student at Aquinas College, was on an ottoman with her arms outstretched. Her clothing appeared to have been cut off.

Her body and hair were covered in blood. She had deep lacerations, including a stab wound to the front of her neck.

The words, "My Bad," were written in blood on a wall.

During Monday's brief court hearing, Joshua Keyzer offered no explanation for the killing. He has undergone two psychiatric examinations and was found competent to stand trial.

His grandmother, Sharon Keyzer, suffered serious injuries in the knife attack but recovered of those injuries. She died of cancer, however, in late December.

John Agar covers crime for MLive/Grand Rapids Press E-mail John Agar: jagar@mlive.com and follow him on Twitter at twitter.com/ReporterJAgar