NEW YORK — A woman fatally shot her ex-girlfriend on a Brooklyn sidewalk, police said.

Nichelle Thomas, 51, was fatally shot at around 1 p.m. Wednesday on Fourth Avenue in Park Slope, police said. Latisha Bell, who police said was the victim's former partner, was arrested on charges of second-degree murder and criminal possession of a weapon.

Surveillance video obtained by  the Daily News shows the 38-year-old shooter running up from behind as the victim opens the door to a deli. The attacker then draws a pistol and shoots the victim in the back of the head.

Police said Thomas was taken to New York-Presbyterian Brooklyn Methodist Hospital, where she was pronounced dead.

Deli worker Mohammed Ali told the newspaper that Thomas had been a regular customer for years. "When I went outside, I saw the blood coming from her head," Ali said. "I know the lady. She was a very nice person. She was always here. I've known her a long time."

It wasn't clear if Bell had an attorney who could comment on the charges against her.

The homicide was the first of the year in Park Slope, a neighborhood with one of the lowest rates of violent crime in New York City.