EVERETT — Prosecutors on Wednesday charged an Everett teen with first-degree murder, alleging that the boy planned last week’s beating death of his guardian, Gina Latshaw.

Latshaw, 37, was bludgeoned with a dumbbell while she slept in her south Everett apartment. Police found her body Friday afternoon after 16-year-old Brad George called 911, reporting that the front door to their apartment was open.

Prosecutors allege that George tried to cover up his involvement and staged the apartment to appear as if someone had broken in.

Later, George allegedly confessed to killing Latshaw, police said. He told police he tried to “clear his head of negative thoughts,” including thoughts he had of Latshaw poisoning him with bleach, Snohomish County deputy prosecutor Paul Stern wrote in court papers.

George reportedly told Everett police detectives that he used a coat hanger to disable a recently-installed security bar on Latshaw’s bedroom door. He allegedly admitted that he stood over the sleeping woman for “14-16 seconds” before he hit her repeatedly with a plastic-covered dumbbell. Officers later recovered a dumbbell in a garbage can outside the apartment. A matching weight was found in George’s bedroom, police reported.

George is expected to be arraigned on the murder charge Thursday in Snohomish County Superior Court.

Latshaw was George’s legal guardian and also the woman he called “Mom.” She spent years advocating for George and trying to obtain adequate treatment for the boy’s mental health disorders, according to court papers.

The boy’s history of mental health issues likely will be raised as the criminal case against him moves forward. Under the law, teens 16 or older charged with a serious violent offense, such as murder, are tried as adults.

Court papers show that Latshaw sought treatment for the boy as early as age 5. Records indicate that the boy was diagnosed with mood and conduct disorders and post-traumatic stress disorder. He told detectives that he had stopped taking his medications about a week prior to the killing.

George also has a history of violence.

In 2010, while he was getting a mental health evaluation in response to reported hallucinations, George broke the arm off a wooden chair and threatened a nurse with it, court papers said.

Last month, George completed probation for a fourth-degree assault conviction in King County. The incident happened in January 2013 while he attended Overlake Speciality School in Bellevue. The school is affiliated with Overlake Hospital Psychiatric Services and serves students who have been diagnosed with behavioral and emotional issues.

George, then 15, punched a wall at the school and kicked a locker. He then shoved a female staff member, causing her to fall back into a locker, according to court papers.

Another staff member confronted George, who ran back toward the victim and pushed her again.

George pleaded guilty in April and was given probation and ordered to do some community service. His supervision was moved to Snohomish County, presumably because he lived in Everett.

Diana Hefley: 425-339-3463, hefley@heraldnet.com