Las Vegas police are searching for two more suspects after 17-year-old high school student Jonathan Lewis Jr was beaten to death.
The teen was caught in a fight in an alleyway near his school, where as many as 10 of his classmates swarmed him.
Eight other suspects aged between 13 and 17 have been arrested and face murder charges, police said.
The dispute was over a set of headphones and a vape stolen from the victim or his friends, they added.
A video shows that soon after Jonathan and another student started fighting, the larger group of students got involved dragging him to the ground.
The teens began "kicking, punching and stomping him, our victim Jonathan, as he is on the ground", Las Vegas Metropolitan Police's homicide lieutenant Jason Johansson told a news conference on Tuesday.
Mr Johansson declined to show the video to the press, describing it as "very graphic" and "devoid of humanity".
The brawl took place on 1 November in an alleyway behind the group's school, Rancho High School. A member of the public found Jonathan unconscious and brought him back to the school. He was later taken to hospital after school staff performed CPR on him.
He died six days later after suffering "non-survivable head trauma", according to Las Vegas law enforcement officials. An autopsy characterized the death has homicide by blunt force trauma.
Eight students were identified and arrested over Jonathan's death, according to Las Vegas Metropolitan Police Department. Two others who have not yet been identified will also face murder charges like the others, the department added.
The police shared a video that included images of the two suspects.
The 17-year-old's father, Jonathan Lewis Sr, said that his son "tried to help a smaller child who was being bullied". But instead, he adds, "15 people attacked him in cowardly violence".
"Our beloved son was beaten to death," Mr Lewis added in a statement.
In a memorial website, he described Jonathan as a "hero".
Scenes from a video of the attack, which Las Vegas undersheriff Andrew Walsh described as "very disturbing", have circulated widely online.
In the same news conference Mr Johansson asked the public to share any additional videos they might have with the police.
"There's a high likelihood that there's additional video out there that we don't have," he added.
Clark County School District (CCDS) officials said in a statement obtained by US media that they are helping the investigation and it "does not tolerate violence or threats to students or their safety". The CCDS represents students across the county, including Las Vegas.
