Just before midnight Saturday, a 17-year-old boy ran down the streets of his suburban Phoenix neighborhood, cradling two younger siblings in his arms. He banged on neighbors’ doors, calling for help. Blood stained their clothes.

Minutes earlier, their 16-year-old brother allegedly attacked his four siblings with a knife, police said, injuring his 9-nine-year-old sister and fatally stabbing his 3-year-old brother in a killing called “gruesome” by local law enforcement.

Alerted by the fleeing siblings, neighbors called 911. While they waited, they treated the injured girl’s wounds while calming the two uninjured children, 17 and 4 years old, with snacks and phone games, reports  The 3 Republic. The 3-year-old never made it out of the house.

When deputies from the Maricopa County Sheriff’s Department arrived at the home, the 16-year-old suspect — covered in his siblings’ blood — ran from the home and led deputies on a quarter-mile foot pursuit before he was tackled and arrested, Maricopa County Sheriff Joe Arpaio said in a news conference Monday afternoon.

Maricopa County Sheriff Joe Arpaio said Monday, Aug. 9, that a 16-year-old suspected in the fatally stabbing of his 4-year-old brother, was spotted running from his home and led deputies on a quarter-mile pursuit before they were able to tackle and arrest him. (CBS News 5)

“It was a vicious, vicious, horrific murder of a 3-year-old,” Arpaio said.

The injured 9-year-old was taken to a local hospital and was expected to recover, Arpaio said. The department has not released the names of any of the children because they are minors.

The 16-year-old remained in a juvenile detention center Monday evening on suspicion of second-degree murder, aggravated assault and endangerment, Arpaio said. The sheriff said he was “confident” the teen would be tried as an adult, but the county’s district attorney has yet to make that decision.

The children’s mother, at work the night of the attack, didn’t learn of the horror until she returned home and found her home turned into a crime scene, a neighbor told The Arizona Republic. The family had lived in the home on Longley Lane for about six years, neighbors said.

“She had no idea,” the neighbor said. “She had to come up the street to see what was going on before she was told what had happened.”

The suspect had no known record of mental illness and there had been no prior calls to the Youngtown home, Arpaio said. Deputies executed a search warrant of the home Monday, but Arpaio declined to divulge any further details about what investigators found there.

Neighbors built a makeshift memorial outside of the family’s home.

The killing was the third fatal domestic violence incident the sheriff’s department has investigated in the past 30 days, Arpaio said, bringing the total number of dead to six.

“It looks like domestic violence in out of hand,” Arpaio said. “I think people in this county and across the nation want some peace of mind, but when you look at what happens in homes and on the streets I can understand their concern.”