A hoverboard theft involving warring factions in a Bloods street gang led to the fatal October shooting of 12-year-old Dejah Joyner — struck in the head as she ate dinner in her Hempstead home, police said Monday.

Jakwan Keller, 20, of 130 Mason St. in Hempstead, fired the bullet into the Dartmouth Street home — striking and killing Dejah — in retaliation for someone stealing a hoverboard from his younger brother, police said.

Keller, who belongs to the Park Side faction of the Bloods, thought Dejah’s older brother stole the hoverboard, said Nassau acting Police Commissioner Thomas Krumpter.

Krumpter would not name Dejah’s brother but said he is a member of the Blood’s High Point crew. He could not “say definitively that Dejah’s brother was involved” with the theft.

“This is a heinous act committed by a coward,” Krumpter said of the shooting, explaining that investigators believe Keller is “the sole person responsible” for the shooting — but adding the investigation is continuing.

“This should be a message to the members of the gangs in this county,” Krumpter said. “These violent, heinous acts will not be tolerated.”

Keller was driving a 2013 BMW at 9:30 a.m. Sunday when police arrested him at the corner of Laurel and Linden avenues, a half-mile from his home, police said.

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At his arraignment Monday in Hempstead’s First District Court, Keller entered the courtroom under heavy security and wearing a bulletproof vest. He pleaded not guilty to charges of second-degree murder, criminal use of a firearm in the first degree and criminal possession of a weapon in the second degree.

District Attorney Madeline Singas attended the emotionally charged arraignment in a courtroom packed with family members of both Keller and the victim as well as a large contingent of court officers.

Keller was represented by a Legal Aid attorney.

District Court Judge Darlene Harris ordered him held without bail. He’s due back in court Wednesday.

As the handcuffed Keller exited the courtroom, family members called out to him, one shouting: “I love you, J!”

Others swore and yelled in protest and court officers attempted to defuse the tensions. Everyone eventually left the courthouse without incident and there were no arrests, court spokesman Daniel Bagnuola said.

Assistant District Attorney Mitchell Benson told the judge Keller had been granted youthful offender status for a previous felony arrest, which seals the record.

According to a Newsday report at the time, Keller was arrested in late 2012 after he allegedly crashed a stolen car into a police vehicle. He was charged with reckless endangerment, criminal mischief, resisting arrest and other offenses, according to the Newsday report.

In 2014, Keller filed a federal civil rights suit against Nassau County, Hempstead Village and several officers from the county and village police departments, alleging excessive force during the 2012 arrest.

“We believed then and we believe now the court should not have confidence in this defendant,” Benson said. Asked if he was concerned about the possibility for more violence among the warring gangs, Krumpter said: “We’ll work to the best of our ability to make sure there isn’t retaliation.”

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Police received 43 tips to the Nassau Crime Stoppers hotline offering a $75,000 reward — more than 20 providing credible information leading to Keller’s arrest Sunday. Police are still working to determine if anyone will collect the reward.

Dejah’s aunt, Carol Miles, wearing a T-shirt and button bearing the little girl’s likeness, said afterward that seeing Keller in handcuffs didn’t necessarily ease the pain of Dejah’s loss.

“Our emotions are basically all over the place at this time,” Miles said. “It was a little crazy in there. We are OK. . . . Listen, this is the fact of the matter. The community itself has been ripped apart by two families. We’re praying for peace for his family. That young man has family just like . . . Dejah does. So we’re praying for peace. We’re not looking for any kind of problem to go back and forth.”