The shooting happened in the officer's parked van, near Stanwood City Hall. Killed was Jenna Carlile, 7, of Camano Island.
Her father, Derek Carlile, has worked patrol for the Marysville Police Department for about three years.
The officer and his wife were nearby at the time of the shooting.
The family is in great pain, as is the police department as a whole, Marysville Police Chief Rick Smith said Monday evening.
"It is tragic," he said. "It is extremely difficult."
The Snohomish County sheriff's Major Crimes Unit is investigating
The case is expected to take some time, sheriff's Lt. Rob Palmer said Monday. Detectives have repeatedly declined to share further details.
"We're fully into investigation mode," Palmer said.
Police also wouldn't say if the weapon involved was the officer's duty handgun or a privately owned firearm.
Derek Carlile has been placed on paid administrative leave until the sheriff's investigation is complete, Smith said.
Marysville police also expect to conduct an internal investigation to determine whether any policies were violated.
Jenna Carlile died from a gunshot wound to the torso, according to the Snohomish County Medical Examiner's Office. Her death has been ruled a homicide, meaning it was caused by another's actions.
That's a medical determination. It is not a legal finding regarding the circumstances surrounding the girl's death.
The girl's sibling, whose age and gender still have not been made public, found a loaded handgun in the vehicle and fired it, investigators said.
Jenna Carlile was taken to Providence Regional Medical Center Everett and then transferred to Harborview Medical Center in Seattle, where she died the next morning.
The chief thanked the staff at both hospitals for their "outstanding efforts" to stabilize the girl and treat her wounds. Jenna Carlile fought hard for her life but eventually succumbed to her injuries, Smith said.
Her family has a strong support system from relatives, their church and the law enforcement community, Smith said.
Jenna Carlile was the oldest of four children, school officials said. She attended Utsalady Elementary School on Camano Island.
Stanwood-Camano School Superintendent Jean Shumate released a statement Monday.
The district sent extra counselors to the school on Monday to work with students and staff, who were "shocked and saddened," she said. The counselors will stay as long as needed.
From Stanwood City Hall on Monday, Mayor Dianne White said she saw a steady stream of people bringing flowers to the shooting scene.
She hadn't heard of any plans for a public memorial, she said.
All donations for the family should be sent to the Marysville Police Department, White said.
"(Carlile) is an excellent officer who is very proactive and loves his community," the police chief said Monday.
According to a profile in The Marysville Globe, Derek Carlile originally is from the Bothell area.
He was thanked in a letter to the Herald in September for his kindness to the family of a young woman killed this summer by a drunken driver.
Police on Monday declined to say whether they expected the results of the sheriff's office investigation to be forwarded to Snohomish County prosecutors for review.
Rikki King: 425-339-3449; email@example.com
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