DEL REY ( — An 8-year-old boy was in critical condition Monday after being struck in the head by a stray bullet while he was sleeping in his Del Rey home.

The incident occurred just before midnight in the 11900 block of W. Wagner Street near Inglewood Boulevard, according to officials.

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“The father was transferring the 8-year-old from his bedroom to the child’s bedroom when he heard the front door open. At that point, he assumed it was his 20-year-old son,” Sgt. Frank Preciado said.

Police say it was an armed suspect who walked into the home through an unlocked door, got into an argument with the boy’s father, began following him through the house and fired at least 10 shots.

The father told investigators he then realized some of the gunshots had gone through the wall to where the boy was sleeping.

“He was saying, ‘I have children in my room.’ It’s heart-wrenching,” Preciado said. “We don’t know exactly what transpired between these two adults, but the children are actually innocent.”

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The boy was shot once in the head. He was rushed to a local hospital where he underwent surgery. He was last listed in critical but stable condition.

Police said Monday the older home’s thick walls may have saved the boy. Neighbors say the family has lived in the residence for more than a decade.

Numerous witnesses were being interviewed. The suspect is described as a heavier set male who was wearing a grey hoodie at the time of the incident.

Police say the family does not know the suspect.

“He’s a good kid… a good student,” the child’s grandfather, Juan Bernal, told CBS2/KCAL9. “I felt bad, really, really bad. I’ve been crying all night.”

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The motive is under investigation.