SEATTLE (AP) — A former University of Washington student who killed his girlfriend in what authorities describe as a violent, drug-fueled rage will spend nearly two decades in prison.

Casey S. Henderson, 23, received the sentence of 18 years and four months Friday in King County Superior Court for the killing of Katy Straalsund.

Henderson pleaded guilty last month to second-degree murder domestic violence. Straalsund, 22, was also a student at the University of Washington.

Police reports say on the night of March 18, 2016, Henderson had been up high on LSD and became convinced Straalsund was plotting against him before he attacked her. The two had been dating for about six months.

Straalsund suffered brain damage after being punched, kicked and strangled. She died several days later.

Henderson apologized for the killing in court.

"I admired her in many ways," he said. "We were always smiling and laughing, sharing some of the best times of our lives together. I just wanted to make her happy. Now she's gone, and I wish it was me instead."

Straalsund's father, Eric Straalsund, said he didn't know how long the attack on his daughter lasted until the sentencing hearing.

"For 20 minutes, she was suffering like that," he said. "This is a monster here. This is not someone who just snapped. It went on and on. My girl was 4-foot-8. She was just tiny."

Superior Court Judge Veronica Alicea-Galvan sentenced Henderson to the top end of the sentencing range for killing Katy Straalsund.

"Despite her young age, she has left an extraordinary legacy of love, bonding, courage and strength," Alicea-Galvan said.