EVERETT, Wash. -

The 16-year-old Everett boy accused of killing the woman he called mother pleaded not guilty Thursday to a first-degree murder charge.

Snohomish County prosecutors plan to try Brad George as an adult in the death of his guardian, 37-year-old Gina Latshaw.

The Daily Herald reports she was beaten with a dumbbell Feb. 7 as she slept.

Court documents say Latshaw spent years advocating for the boy and trying to obtain adequate treatment for his mental health disorders.