DISCOVERY BAY (CBS SF) — Prosecutors Tuesday filed first-degree murder charges against a teen accused of fatally stabbing a 9-year-old boy in Discovery Bay on Sunday morning, according to the Contra Costa County District Attorney’s Office.

William Shultz, 18, has been charged with murder with the use of a knife and the special circumstance allegation of lying in wait, prosecutors said. He is also charged with one count of residential burglary.

READ MORE: UPDATE: Atmospheric River Crashes Onshore; Driving Rains, Gusty Winds, Flash Flood Fears

Shultz is accused of fatally stabbing Jordan Almgren in Jordan’s Discovery Bay home. Sheriff’s deputies were called to the home at 10:09 a.m.  Sunday on reports of the stabbing and learned Jordan had already been taken to the hospital, where he was pronounced dead.

Deputies quickly identified Shultz as a suspect in the case. A nurse at Kaiser Permanente’s Antioch Medical Center saw social media reports indicating Shultz was wanted, realized he was at the hospital and alerted deputies at about 3 p.m. He was arrested shortly afterward, according to the sheriff’s office.

Deputies had encountered Shultz a day earlier when his family asked for a mental health evaluation. The deputies determined he did not meet the criteria for an involuntary mental health hold, but he agreed to be taken to the hospital, according to the sheriff’s office.

But he did not stay at the hospital long and that night was staying at Jordan’s family’s home on a sleepover. Investigators believe he attacked Jordan sometime in the early morning hours.

READ MORE: Woman Dead in Horrific Crash on I-680

Sheriff’s officials had not established the motive for the stabbing as of Monday.

Jordan was a student at Timber Point Elementary School. The Byron Union School District is providing counseling services for students this week to assist with the grieving process, district superintendent Debbie Burnette said Monday.

The school is also providing chaplains to lend support to adult staff members, Burnette said.

The stabbing remains under investigation. Anyone with information about the case is asked to contact sheriff’s detectives at (925) 313-2600.

MORE NEWS: Stockton Man Sentenced to 7 Years in Prison for Shooting of Drug Dealer

© Copyright 2015 by CBS San Francisco and Bay City News Service. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.