A young boy fled his southwest Denver home Thursday night to a neighbor’s house so he could call 911 and report his mother, father and sister had been shot.

All three of his family members died in the shooting, police said. A fourth adult received nonlife- threatening injuries.

Denver police arrested the man suspected of the killings, Bustaman Kartabrata, early Friday morning in a northeast neighborhood, said Joseph Montoya, Denver police investigations division chief, at a Friday afternoon news conference.

Kartabrata, 59, remains in custody on suspicion of three first-degree murder charges. Kartabrata knew the family, but it’s not clear what precipitated the shooting, Montoya said. There was no forced entry into the home.

“We do know that they were very well acquainted,” Montoya said.

The victims’ names have not been released. Denver Public Schools said the deceased girl attended Traylor Academy, an elementary school.

“This is heartbreaking, and my thoughts go out to everyone touched by this tragedy, including the family,” Denver Public Schools Superintendent Susana Cordova said in an emailed statement. “Please continue to keep them in your thoughts.”

The woman was pronounced dead at the scene on the 3000 block of South Golden Way. The man and the girl were taken to a hospital, where they were pronounced dead, police said.

The two-story home is located near Bear Valley Park and had a for sale sign in the front yard along with a red Dumpster in the driveway. Neighbors told The Denver Post the family, which city records show had lived there since 2008, was planning to move to Mississippi at the end of the school year. The cost of living in Denver, they said, was getting to be too much for their neighbors.

“They were moving next week, I think,” Debbie Ivey said. “They were just real excited that she wasn’t going to work three jobs and about buying a new house.”

Denver police confirmed that the family was closing the sale on the house.

Ivey and her husband, Tom, described the family as kind, friendly and hardworking, with a daughter in fifth grade and a son in third grade. The Iveys said the couple had been working almost every day to get the house ready for its new owners and had recently started sleeping at a nearby hotel where the woman worked.

“It’s just so tragic, because they were right on the cusp of having a really good life in Mississippi,” Tom Ivey said.

Debbie Ivey said a police officer came to their home after the shooting to ask for food and water for the boy.

“I’ve never see a policeman that big, that shaken,” she said.

Investigators continued to investigate the suspect’s motive, Montoya said. He would not confirm which kind of gun was used or how the young boy escaped the shooting.

Kartabrata did not have any previous criminal records beyond traffic tickets, court records show.

“In all my time as a police officer, this is one of the most emotional incidents I’ve ever dealt with,” Montoya said.

“It’s something that didn’t need to happen,” he added. “It’s just so senseless.”

Staff writer Kirk Mitchell contributed to this report.