Police arrest juvenile accused of killing 4 people inside a home

Police in Grantsville, Utah, have arrested a young boy after four members of a family were shot to death inside a home Friday evening, police said.

Officers responded to a call to the residence around 7 p.m. and discovered the victims -- one adult female, two juvenile females and a juvenile male -- with bullet wounds.

"We believe all the victims are related," Grantsville Police Cpl. Rhonda Fields told ABC News on Saturday.

The alleged shooter, a juvenile, left the home with an adult male before the police arrived, Fields told reporters early Saturday morning.

The suspect and the older male were heading to a nearby hospital, where adult was listed in stable condition with a gunshot wound, police said.

Utah Governor Gary Herbert took to Twitter to send condolences and urged "parents and grandparents [to] secure your firearms."

Police said the juvenile was the lone shooter and that he was taken into custody without incident.

Fields told ABC News the investigation is ongoing and that because a majority of the victims are juveniles, their names won't be released at this time.

This is the 10th mass shooting in which four or more people have been killed or injured in the U.S. in 2020, according to the Gun Violence Archive, a not-for-profit that tracks shootings.

Definitions on what constitutes a mass shooting vary among organizations, but the FBI generally defines such incidents as those in which four people, excluding suspects, are killed.

ABC News' Alexandra Faul contributed to this report.