Teen charged in shooting deaths of 5 family members tried to blame his brother, court documents say

A 15-year-old boy was charged with murder Thursday in the shooting deaths of his parents and three of his siblings at a home in Fall City, Washington, according to court documents obtained by CBS News.

The teen, whose name is being withheld because he is a juvenile, was charged with five counts of aggravated murder in the slayings of his parents, Mark and Sarah Humiston, two brothers, ages 9 and 13, and his 7-year-old sister, per King County court records.

He was also charged with one count of attempted murder for shooting and wounding his 11-year-old sister, the documents say.

That girl was in "satisfactory condition" at Harborview Medical Center in Seattle, hospital spokesperson Susan Gregg told The Associated Press on Tuesday.

Autopsies performed by the King County Medical Examiner's Office determined that all five victims died of gunshot wounds. The handgun used in the shooting was believed to belong to the victims' father, the court records state.

According to the charging documents, just before 5 a.m. Monday, the suspect called 911 with a false story in which he claimed that his 13-year-old brother "just shot my whole family and committed suicide too" at the family's home in Fall City, a community near Seattle.

However, at about the same time as that call, 911 dispatchers received a second call from a neighbor who lives about a quarter-mile away. That neighbor said the suspect's 11-year-old sister had run to his house and was bleeding from what appeared to be a gunshot wound, the documents said.

The girl said her entire family had been fatally shot and identified her 15-year-old brother as the gunman. The girl told dispatchers she was also shot by her brother and "then described holding her breath and playing dead," the documents read. The girl later told detectives she had escaped through a bedroom window.

Deputies responded to the Humiston home and found the suspect in the driveway and took him into custody, court records show. The five bodies were found inside the home.

In a hospital interview with detectives later that day, the suspect's surviving sister said she identified the firearm used in the shooting as "her father's silver Glock handgun," court documents state.

She said her father kept the pistol in a small lockbox that "he would sometimes put by the front door so he could bring it to work," the documents read. She told detectives the suspect was "the only one who knew the combination to the Glock lockbox."

Investigators determined that the suspect "systematically murdered" his parents and siblings and "then staged the scene prior to the arrival of first responders to make it appear" that the murders had been committed by his brother, documents read.

The court documents did not speculate on a motive.

The suspect was scheduled to be arraigned Friday afternoon. The King County Prosecuting Attorney's Office said in a news release Thursday that the suspect was being held at the Clark Child and Family Justice Center, which is a facility for juveniles.

Prosecutors are requesting that the case be moved to adult court, CBS affiliate KIRO-TV reported Friday. If transferred to adult court and convicted, the teen would face a sentence of 25 years to life, with the potential for release after 25 years, subject to review by the state's Indeterminate Sentence Review Board.

An arraignment will not take place until the decision of adult or juvenile court is decided, the station reported. That decision could take months.

In a statement Tuesday, public defenders representing the suspect said he "is a 15-year-old boy who enjoys mountain biking and fishing and has no criminal history."

Mark Humiston worked as an electrical engineer with Hargis Engineers in Seattle, The Associated Press reported.

"We are blindsided and saddened by the tragic events that have led to the loss of a respected colleague, mentor, and friend, as well as the loss of immediate family members," the company said in statement Thursday. "Mark's leadership and vision were integral within our firm, and he will be greatly missed."