An 11-year-old girl is believed to be the sole survivor of the mass killing inside a lavish Washington state home on Monday where five family members — including three young kids — were found shot to death, according to local reports.

A 15-year-old boy is behind bars after police found the lifeless bodies of two adults and three young teens riddled with bullets inside the family’s $2 million cabin in Fall City around 5 a.m., while a sixth victim, the girl, was shot twice during the disturbing massacre.

Prosecutors on Tuesday asked the court to charge the teen — who was not identified — with five counts of first-degree murder and one count of first-degree attempted murder, according to the King County Prosecuting Attorney’s Office.

Judge Joe Campagna ordered the alleged killer to remain in secure juvenile detention as he awaits formal charges, calling the allegations the “gravest possible seriousness,” the Seattle Times reported.

Campagna also prohibited any contact with his sole immediate family member who survived the attack, the 11-year-old girl, the outlet reported.

The teen, who waived his right to attend his first appearance in King County Superior Court, was described by defense attorney Amy Parker as a young boy who “enjoys mountain biking and fishing.”

She also pressed that her client has no criminal history.

Authorities have not revealed the identities and ages of the victims, though the child victims are believed to be teenagers, with the eldest around 15, neighbors told FOX13.

Deputies responded to the Lake Alice Road home — a lovely waterfront property — just before 5 a.m. on Monday after receiving multiple 911 calls of a “disturbance with gunfire” and found the five family members shot dead, King County Sheriff’s Office spokesperson Mike Mellis said.

At least some of the 911 calls that alerted deputies to the shooting came from victims who were inside the home and hiding in the bathroom, KIRO 7 reported.

Deputies immediately arrested the teen inside the house, Mellis explained, and the surviving youngster was taken to Harborview Medical Center in Seattle after receiving medical aid from a neighbor following the shooting.

As of Tuesday, the King County Medical Examiner’s Office had not released the identities of the victims who were killed.

Prosecutors are expected to file formal charges on Thursday, with an arraignment to follow on Friday.