The 15-year-old boy accused of killing his parents and three siblings in the family mansion in Washington state could spend less time behind bars if his case does not go to adult court, prosecutors said.

The unidentified teenage suspect can only be held until age 25 — more than 10 years — if convicted in juvenile court, according to the King County Prosecutor’s Office.

Prosecutors argued Friday that the horrific case should proceed to adult court teenager faces life in prison for allegedly shooting his parents – Mark, 43, and Sarah Humiston, 42 — his two brothers and sisters, ages 7, 9 and 13, died and injured his 11-year-old sister in a bloody massacre on Monday.

Defense attorney Amy Parker described the alleged teen killer as a caring boy with no criminal record who loved mountain biking and fishing.

“We want the court to know that several people in the community have contacted us who know our client and have attested to his good character,” she said in court Friday.

“He was described by them as kind and caring. This is a boy who has many friends, has pro-social interests and has contributed positively to his community.”

Deputies responded at the Lake Alice Road home Monday around 5 a.m. and found the victims’ lifeless bodies covered in blood and riddled with bullets to the face, head and body in the family’s $2 million Fall City home, according to charging documents obtained by the Post.

The teenager is accused of hatching a disturbing plot to stage the massacre as a murder-suicide by his 13-year-old brother and be the family’s sole survivor, according to charging documents.

The report claims that the 15-year-old tried to say that his brother, who was killed, committed the crime after he was caught watching porn the night before.

Detectives also say the accused teenage killer tried to stage the crime scene to make it look like his brother committed the heinous act and “put” a gun in his hand after he was killed.

The The suspect’s 11-year-old sister, the only family member to survive the deathsaid her brother used her father’s silver handgun to fatally shoot her parents, her brothers Benjamin and Joshua and her younger sister Katheryn, according to charging documents.

According to the documents, the teenager’s murderous rampage came after he failed some tests at school and got into “a lot of trouble”.

The young girl was sleeping in her room when she woke up to the deafening sound of gunshots.

She tearfully told investigators that her older brother came into her bedroom and allegedly opened fire on her, hitting her in the hand and neck and killing their younger sister, who shared a room.

The girl then “pretended to play dead” while her brother reentered the room and stood by her bed before he reappeared, according to charging documents.

She fled home through her bedroom window when she felt her brother wasn’t going to catch her and ran to a neighbor’s house, where she called the police.

Deputies responded to the family’s home, about 30 miles outside Seattle, after receiving 911 calls from the alleged killer and his surviving sister, according to the documents.

injured girl who was taken to Harborview Medical Center following the massacre, he identified his 15-year-old brother as the shooter.

Deputies immediately arrested the juvenile after discovering the bodies and his sister identified him as a suspect, prosecutors said.

The teenager was charged with five counts of first-degree murder and one count of first-degree murder and is being held at the Clark Child and Family Justice Center in Seattle.

A judge is expected to make a decision on which court the teenager will be tried during a hearing on Jan. 8, 2025, prosecutors said.