A 16-year-old high school student fatally stabbed in a fight with other youths over a girl in an Oceanside strip mall lay dying on the sidewalk as dozens of teens recorded the stabbing on their smartphones instead of helping, Nassau police said Tuesday.

The slain boy, identified as Khaseen Morris, was stabbed once in the chest on Monday afternoon in the parking lot of the mall, where students from nearby Oceanside High School congregated before and after school. Morris, who would have turned 17 next month, died just before midnight Monday at South Nassau Communities Hospital, police said.

"Kids stood here and didn't help Khaseen," said Nassau Det. Lt. Stephen Fitzpatrick, commanding officer of the department's Homicide Squad. "They'd rather video. They videoed his death instead of helping him."

The victim's older sister wept as she recalled watching a social-media video that showed her brother bleeding on the sidewalk.

"He was in so much pain," Keyanna Morris, 30 of Oceanside, said in an interview.

Some 50 to 70 youths were at the strip mall at the time of the stabbing, and many of them, according to police, whipped out their smartphones and recorded Morris being stabbed and bleeding profusely as he lay on the sidewalk. The students later circulated the footage on social media including Snapchat.

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Fitzpatrick said police identified the assailants through witness statements and the video, as well as surveillance footage from nearby businesses. Police are looking for six to seven youths believed to have been involved in the melee.

He also issued a striking warning to teens who were involved or at the scene, telling them they should come forward with their version of events.

"The individuals who are responsible for this, if you're not a part and parcel to the murder of Khaseen Morris, now's the time to get in touch with us and let us know who did this and why," said Fitzpatrick. "If you were just coming here and thinking you were fighting, and then he got stabbed during that, you need to be out in front of that. After I put handcuffs on you, that ship has sailed."

Morris, a senior at Oceanside High School who was an avid skateboarder and described by family as artistic and bubbly, had received texts advising him that another youth wanted to fight him over a "perception" that he was dating a girl who may have had a past or current relationship with one assailant. Fitzpatrick said investigators are still trying to determine if, in fact, Morris was dating the girl.

Morris was unarmed at the time of the attack, Fitzpatrick said, but he knew that the ex-boyfriend was looking to confront him.

"Yesterday, apparently there was a prearranged type of dispute going on over a young lady," he said. "That dispute boiled over here, where a group of six to seven males charged at the victim and his couple of friends and, during that melee, stabbed him."

Keyanna Morris said her brother told her he went to a party on Sunday and walked a girl home from there at her request.

Keyanna Morris said the girl’s ex-boyfriend sent her brother "threatening messages" in response, echoing what Fitzpatrick said.

"He told me that someone was threatening him," said Keyanna Morris. "But he's like, ‘I'm not worried about it. I told him, I don't want anything ... I don't want your girl. It's nothing like that. I just walked her home.’ ”

Keyanna Morris confirmed the police account that her brother knew the boy wanted to fight him, and said while her brother wasn't looking for a fight, he wasn't going to run away either.

"He knew the one boy wanted to fight him," she said. "Khaseen is like, ‘OK, if you beat me up, fine.’ ”

But, she added, "No one knew he was going to bring a million people out there — and a knife."

The attack also sent a second boy, a 17-year-old who was one of Khaseen Morris’ friends, to a hospital with a broken arm and swelling to his head.

Phyllis S. Harrington, superintendent of schools in Oceanside, said Tuesday morning the district has counselors ready to help students and faculty members.

At the strip mall on Tuesday morning, business owners and patrons expressed shock and sadness.

Joel Kim, the owner of Ace Cleaners, said he was in the back working when he heard a commotion and saw the victim bleeding profusely on the ground in front of his storefront. He said he gave water and paper towels to teens who were trying to help. "I did the best I could," he said.

The strip mall, with a variety of businesses including a bagel store, nail salon, Chinese restaurant, hair salon and a pizzeria, is a frequent hangout spot for students from nearby Oceanside High School, business owners said.

Deepak Shah, who has owned Heart To Heart, a stationery and card store in the plaza for 22 years, said he called 911 after the stabbing.

“People are upset,” Shah said Tuesday. “All my customers are regulars. Nobody wants to see that happen.”

With Joie Tyrrell