A teenager near New York City was stabbed to death in an attack witnessed by dozens of onlookers — some who rolled video of the slaying rather than help stop it, police said Tuesday.

Khaseen Morris, 16, and a friend were set upon by as many as a half-dozen people Monday in an after-school brawl in front of a pizzeria in a strip mall parking lot in Oceanside, a Long Island community about 30 miles east of Manhattan, authorities said.

Morris was stabbed in the chest, and a 17-year-old friend suffered a broken arm and swelling to his head, police said.

Authorities announced an arrest in the fatal stabbing on Wednesday. Tyler Flach, 18, was charged with murder in connection with the death of Morris, Nassau County police said in a statement. He will be arraigned Thursday.

The death of the teen was witnessed by dozens of onlookers — some who recorded the slaying rather than help stop it, police said Tuesday.

Nassau County Det. Lt. Stephen Fitzpatrick told reporters the attack appears to have stemmed from a dispute over a girl "and the perception of who she might be dating or who she might be hanging with."

Investigators believe at least 60 teens had gathered around the parking lot to watch the confrontation, and some took cellphone video that was posted on social media.

“Kids stood here and didn’t help Khaseen,” Fitzpatrick said. “They’d rather video. They videoed his death instead of helping him.”

The detective appealed to onlookers to come forward.

“If you’re not a part and parcel of the murder of Khaseen Morris, now’s the time to get in touch with us to let us know who did this and why," Fitzgerald said. "If it was just coming here and thinking you were fighting and then he got stabbed during that, you need to get in front of this."

A teen boy has died and another was still in serious condition after cops were called to Mario's Pizzeria and Restaurant in Oceanside, N.Y. Monday after reports of a large fight. NBC New York

Khaseen, who had just started his senior year at Oceanside High School, walked a girl home from a party on Sunday night, a chivalrous act that led to his slaying, family members said.

"The girl told her ex-boyfriend and said that, 'Oh another boy walked me home,' and she kind of set up this thing to get him jealous," an older sister of Khaseen's told NBC New York.

Nassau County Executive Laura Curran said Wednesday that additional police officers will patrol Oceanside High School and neighboring schools and that the county reached out to the Oceanside School District to ensure they have the mental health resources to support students in need.

"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," Curran said in a statement.