A gang-banger arrested after his correction-officer ex-girlfriend was fatally gunned down in Brooklyn was finally charged with her killing Thursday, police said.

Alastasia Bryan, a 25-year-old newly minted correction officer, was killed while sitting in her car Dec. 4 in Flatlands, police said.

Keon “Trini” Richmond was arrested 10 days later in the same neighborhood on charges that he torched a car belonging to another ex-girlfriend, cops said.

Cops said at the time they were eyeing him in the killing. He now faces charges of murder, criminal possession of a weapon and criminal use of a firearm.

Richmond fired five bullets at Bryan, striking her in the head and torso, killing her instantly, cops said. He then hopped into his car and sped away.

Richmond — a high-ranking member of the Pay Back Crips street gang who has 30 prior arrests — was believed to have been seething because Bryan had him busted for stealing her leased 2010 BMW 750 after he took it and refused to give it back, police sources said.

Cops arrested him on Nov. 2 and charged with grand larceny auto. He was supposed to be in court for a hearing on the incident in January.

Bryan had also filed a report against her ex in May 2015, accusing him of making death threats against her.

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