She had been his legal guardian for 10 years.
Ms Georgina Latshaw, 37, was found battered to death in her bedroom after the US teen returned home from school in Washington State. Brad George, 16, suffers from several disorders, including post-traumatic stress disorder.
In a chilling interview to Kiro TV, given with his two sisters, he said: "I don't know who could have done this. This doesn't feel real, this all feels fake."
Now, police suspect he was the killer, the Mail Online reported.
They believe the killing was premeditated because of the number of steps he went through to allegedly cover up his involvement.
After returning home from school on Friday afternoon, George called 911 and said he had become alerted to the front door lying ajar with his mother's car still outside.
Officers found her dead in her bed with blunt-force trauma to the head, according to the police report.
Investigators then discovered a purple plastic dumbbell and male clothing stained with blood in the garbage and no sign of forced entry or theft.
The apartment was searched with a judge's permission and an apparent twin to the purple dumbbell was found in the teenager's bedroom. When they confronted him, the teen denied any involvement but was reportedly nervous. He began to shake when questioned about the dumbbell.
He told the police that he had broken into her room which had a security lock and then waited until she began snoring. He then hit her head with the dumbbell at least three times.
He also told the police that he had not been taking medicines for his disorder since Feb 1 and had not slept in three days.
Police said the teen was arrested on Saturday and taken for an evaluation at a psychiatric hospital where he suddenly appeared to be in a daze.
Court documents showed that he once threatened Ms Latshaw with a knife and had hallucinations of her as "a short clown with a blood filled mouth".
He also made a clay sculpture of Ms Latshaw with a severed head.
He now faces charges of first degree murder.
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