YAKIMA -- In the end, Chelsie Stoneking was not pregnant.

But a mistaken belief that she was apparently contributed to an attack by her on-again, off-again boyfriend that left the 16-year-old dead, an act Yakima County Superior Court Judge Blaine Gibson described as incomprehensible in its savagery.

Gibson sentenced 19-year-old Heriberto Saucedo last week to 23 years in prison for his guilty plea to first-degree murder for the Dec. 20, 2009, killing at Stoneking's Grandview home.

Stoneking suffered severe head trauma, strangulation and 57 stab wounds; some were defensive.

Gibson said Saucedo's immaturity and frustration over his struggles in school don't justify the actions that resulted in the taking of a life.

Saucedo was 17 at the time of the murder.

In handing down the sentence, Gibson accepted the prosecution's request for 276 months.

The sentence was the result of a plea agreement that was reached because of prosecutors' concern about their ability to prove premeditation, said Deputy Prosecutor Gary Hintze.