Two other boys who had been wooing the same girl agreed to meet outside Stansbury Elementary School just before midnight to settle things once and for all, police said.
Horowitz showed up to support a friend. He hadn't planned on raising a hand to anyone.
But when a member of the opposing group began lobbing insults and challenges to Horowitz amid the churning mob of teenage machismo, the Stansbury High School senior was drawn into a fight he would not escape alive.
Witnesses told police that Horowitz was stabbed several times in the chest by a young man with a knife.
He died on the spot.
Tooele County Sheriff Frank Park said deputies received a call about the stabbing about 12:11 a.m. Saturday.
By the time they got to the scene, Park said, the individuals believed to be responsible for the slaying had fled.
Officers from the Sheriff's Office, the Utah Highway Patrol and Tooele City police rounded up about a dozen juveniles and young adults in the area to take in for questioning.
By 5:30 a.m., investigators had narrowed their list of suspects and executed a search warrant at a home in Stansbury Park, where they believed they would find the alleged killer.
Beach was booked into the Tooele County Jail on suspicion of murder. Coffey was booked into jail on suspicion of obstructing justice and being an accessory to murder.
No charges had been filed as of Saturday night, but the sheriff said they would be forthcoming.
Park said the relationship between Coffey and Horowitz was murky at best they had gone to the same high school for several years but officials did not believe Beach knew Horowitz at all.
"This was just one person attacking another person," Park said.
According to a statement released by Tooele County, Horowitz "was well known throughout the Stansbury Park community and will be deeply missed by all who knew him."
The Sheriff's Office's investigation is continuing.