As suspect is arrested, grandmother mourns 16-year-old girl stabbed to death in Harlem: ‘Why would you do that?’ (EXCLUSIVE)

An 18-year-old man has been arrested for fatally stabbing a 16-year-old girl in Harlem, police said late Monday.

Saniyah Lawrence bled to death after she was slashed in the neck in an apartment building on Adam Clayton Powell Jr. Blvd. near W. 136th St. about 5:40 p.m. Sunday.

Zyaire Crumbley was arrested and charged with murder on Monday night, cops said.

“Why would you do that?” the victim’s grieving maternal grandmother, Denise Jackson, 51, told the Daily News. “Why would you just take her life?”

The mortally wounded girl was found in an apartment after police got a 5:40 p.m. 911 call from someone in the building.

Medics rushed Saniyah to Harlem Hospital, where she died a short time later. She lived in Harlem about a mile from where she was killed.

Crumbley allegedly ran off with the murder weapon. Police said he has four prior robbery arrests, including one for which he has been on probation since May.

The apartment where the slaying occurred is unoccupied, with different people cycling in and out, police said.

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“She’s a very lovable person,” Jackson said. “Everyone that knows Saniyah would never want to hurt her because she’s not that type of person. What was the motive?”

Jackson said her granddaughter often stayed with her because Saniyah’s mother is in and out of the shelter system and her father is not around. She had been dating her boyfriend for about a year.

Saniyah liked to spend time with her friends and shop. Math was her favorite subject in school, Jackson said, and she had hoped to get a part-time job in the new year.

“Put this out: We need justice for Saniyah,” Jackson said. “She didn’t deserve this.”

Meanwhile, the suspect’s 61-year-old grandmother told the Daily News she has no knowledge of her wanted grandson’s whereabouts.

“I’m his grandmother, but I have nothing to do with him,” she said, declining to give her name. “I know all about what happened, but I don’t know anything about him. … I don’t associate with those people.”