A 15-year-old accused of killing his Fall City family tried to frame his brother, according to investigators

Investigators with the King County Sheriff's Office accuse a 15-year-old juvenile of shooting all of his immediate family members in his Fall City home Monday, killing five of six family members. Authorities say the teen also tried to “stage” the scene and blame his 13-year-old brother, telling dispatchers that the younger brother shot others and then committed suicide.

However, his report was directly contradicted by his 11-year-old sister, who was wounded but managed to escape and ask neighbors for help. Charging documents say the surviving sister told investigators from her Harborview hospital bed that her older brother fired a gun at her as she lay in her bunk bed in the early morning hours of Monday.

She heard more gunshots and saw the bodies of family members in the hallway outside her bedroom. Investigators later found the bodies of her mother, father and three siblings, ages 13, 9 and 7.

The girl used the “fire window” in her room to escape. She told investigators that she recognized the gun her brother was carrying “as her father's silver Glock pistol” and that her brother was the only child in her family who knew the combination to the firearm locker. Officers detained the 15-year-old, the oldest of the family's five children, in the driveway.

On Thursday, King County prosecutors charged him with five counts of aggravated first-degree murder. He is also charged with one count of attempted first-degree murder. This charge also includes a firearm enhancement. An arraignment is scheduled for Friday.

KUOW is not releasing the suspect's name due to his age. It is not yet clear whether he will be charged in juvenile or adult court.

The court has barred the suspect from any contact with his younger sister, who was released from Harborview on Tuesday.

On Thursday, the King County coroner confirmed the names and ages of those killed and listed the cause of death for all five family members as gunshot wounds. These included parents Mark Humiston (43) and Sarah Humiston (42) as well as children Benjamin (13), Joshua (9) and Katheryn (7).

KUOW is not releasing the name of the surviving sibling, age 11.

Sheriff's Office Detective Aaron Thompson said in the “certificate of probable cause” attached to the indictment that the evidence showed the 15-year-old “systematically murdered” his family members and attempted to murder his surviving sister , and “staged the crime scene before first responders arrived” to make it appear that his younger brother had committed the murders and then killed himself.

The defendant then “further reinforced the false narrative by repeatedly telling the emergency dispatcher” that his younger brother was responsible. The investigators found the weapon in the hand of the fallen 13-year-old. Dispatchers received emergency calls from the defendant and shortly thereafter from neighbors of the house where the sister had fled.

The sheriff's office said they took the 15-year-old into custody at the home without resistance. At an initial hearing in King County Superior Court on Tuesday, his public defender said the teen had no prior criminal record.

At that hearing, in which the juvenile waived his right to a first appearance, King County Superior Court Judge Joe Campagna called the allegations “of the utmost seriousness” and ordered the juvenile to remain in secure juvenile detention.

Prosecutors said they would ask a judge to move this case to adult court. That would allow for a potentially longer sentence. If the case remains in juvenile court, the 15-year-old could only be held until he turns 25. If a judge agrees to transfer the case to adult court, the sentence set by the state legislature is 25 years to life with a presumption of release after 25 years, according to prosecutors.”

A statement from King County prosecutors said: “A case may be transferred to adult court only with the approval of a judge and after hearing from the prosecution and defense.” Based on the details in the charging documents, prosecutors are asking a judge for one “Hearing where both parties will present evidence and the judge will decide whether to send the 15-year-old to adult court.”

The father, Mark Humiston, worked as an electrical engineer for Hargis Engineers. The company issued a statement this week. “We are surprised and saddened by the tragic events that have resulted in the loss of a respected colleague, mentor and friend, as well as the loss of immediate family members,” the statement said. “Mark’s leadership and vision were an integral part of our company and we will miss him greatly. Our thoughts are with his surviving family, friends and colleagues at this difficult time.”

KUOW's Ann Dornfeld contributed to this story.