A 16-year-old boy was stabbed to death while dozens of teenagers watched or used their phones to film an after-school assault.

Police have charged 18-year-old Tyler Flach with second degree murder following the death of Khaseen Morris on Monday at a mall near Oceanside High School in Nassau County, New York.

Police say 50 to 70 bystanders witnessed the attack but failed to call police or defend Morris. Footage of the brawl was posted to Snapchat and other social media platforms, according to police.

A 17-year-old male also was sent to hospital to be treated for a broken arm.

On Monday afternoon, a fight broke out among a group of seven to eight people who attacked Khaseen Morris in a dispute about a “girl and a perception of who she might be dating or who she might be hanging with”, Nassau County Police Department Detective Lt. Stephen Fitzpatrick said. Police said Flach, a freshman college student, was an “associate” of the girl’s ex-boyfriend.

Morris died at South Nassau Communities Hospital on Monday after being stabbed in the chest.

An attorney for Flach said he expects to plead not guilty. In a statement to Newsday, attorney Ed Sapone said Flach “strongly maintains his innocence” and “took the responsible step of self-surrendering to eventually clear his name”. Mr Sapone says that at “appropriate time,” the defence will release “vital information” not captured on video of the attack. He also asked the Oceanside community to “reserve judgment” until additional information is released.

Tyler Flach (Nassau County Police)

In a video posted to YouTube on Wednesday, Oceanside Superintendent of Schools Phyllis Harrington urged students and parents to “use restraint and judgment” on social media and encouraged parents to monitor their children’s social media usage.

“It’s important to the safety and security of our school that we do not engage in spreading rumours,” Harrington said.

More than 300 people attended a memorial for Morris on Wednesday night, the second of a two-night vigil at the site of the attack, and called for justice.

Police will be assigned to patrol Oceanside High School and neighbouring schools, according to a statement from Nassau County Executive Laura Curran. County authorities also are offering mental health services to support students.

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“I want to plead with all our residents -- not just our young people: If you see someone in serious danger, please use your phone to get help - not likes and shares,” Ms Curran said. “Our law enforcement and first responders are trained to save lives in dangerous situations, and someone’s life can depend on a matter of seconds.”