Police were awaiting autopsy results Monday as they tried to unravel why a boy allegedly shot and killed four people including three children inside a suburban Utah home.

Grantsville Police Cpl. Rhonda Fields told The Associated Press that the victims’ identities would be released following completion of the autopsies, which was expected sometime Monday. The victims were related to each other, investigators believe.

A candlelight vigil for the victims was planned for Monday night at Grantsville City Park. The killings were the first homicides in nearly 20 years in the town of 11,000 about 35 miles (55 kilometers) west of Salt Lake City.

Police responded to a call of shots fired inside the home at approximately 7 p.m. Friday. When officers arrived, they found the bodies of two girls, a boy and a woman, Fields said.

The shooter and the surviving victim were gone, she said. Authorities later discovered that a person who arrived at the house after the shooting drove the suspect and the surviving victim to a nearby hospital, Fields said.

Authorities said they had not been called to the house in the past.

Officers arrested the boy at the hospital. He faces 10 charges including aggravated homicide, Fields said.

He was being held at a youth detention facility and his identity was not released because he was charged as a juvenile.

Officials said he is the only suspect. His relationship to the victims was not immediately clear.

The fifth victim had sustained a gunshot wound, was in stable condition and expected to survive, Fields said. The person who drove them to the hospital was not involved in shooting, she said.

It appears to be the largest mass shooting in Utah since 2007, when a shotgun-wielding gunman killed five people and himself at Trolley Square mall in Salt Lake City.

Tooele County School District officials planned to offer counseling to students when they return to school on Tuesday.