It happened around 12:30 p.m. on City Island Avenue and City Island Road, where police say a 67-year-old man rode up on a bicycle and opened fire.
The woman, who area residents said worked at the site directing traffic, was struck multiple times and later died at the hospital.
Police identified the victim as 52-year-old Brooklyn resident Lizbeth Mass.
The suspect fled on the bicycle, and the victim's boyfriend, who was present at the time, got into his own vehicle and struck the man with his car.
The 58-year-old boyfriend then exited his vehicle, ran after the suspect, beat him up, and held him until police arrived.
He was there to visit the woman on her lunch break.
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Area residents say the woman had been at the construction site for a couple of weeks and was known to be animated while holding a sign telling people to slow down.
"We've come to know her," said one man who didn't want to be identified. "She's here every day, she's waving flags, she's very animated, she has a very bubbly personality, even the people that go for their morning walks have come to know her."
The man told Eyewitness News that a colleague was nearby when the shooting happened, and that the gunman was known to the victim.
"This guy, the guy that shot her, (was) bringing her lunch every day," he said. "So I can't imagine what went awry here, but it's so unfortunate. I pray for her soul. She was a very nice person, very bubbly personality, the last kind of person you'd want to see something like that happen to. It's very sad."
The suspect was taken into custody at Jacobi Medical Center, where he was treated for body trauma.
He has since been arrested and charges are pending, according to officials. They don't believe the incident was domestic-related.
A firearm was recovered at the scene.
The boyfriend was also questioned by police.
Sources tell Eyewitness News that he is not expected be charged.
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The motive for the shooting is unknown, and the investigation is active and ongoing.
"This guy used to bring her sandwiches every day," the man said. "Today, he brought bullets instead."
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