Authorities say 28-year-old Cherokee Fletcher was struck on Motor Parkway and Moreland Road in Hauppauge around 10:15 p.m.
Investigators believe the vehicle collided with another car, before striking her, and driving off.
Her fiancé said she left home to get money at a local 7-Eleven for her children's field trip and never made it back home.
Steven Bartolomey told Eyewitness News that Fletcher was a wonderful mother.
"She did anything for her kids, she went the extra mile," he said. "I only seen maybe once or twice in my life, my mother and grandmother, the motherhood she had for her children, the love she had for them."
They were living at a nearby shelter but had plans to move out and marry this summer.
He says the suspect took all of that and more away.
"Turn yourself in," he said. "Why? Why did you leave her body there? She left five children. Five children. It's not fair. We planned a wedding. It's never going to happen now."
About 15 minutes later in nearby West Islip, police said a car struck 32-year-old Tracey Dorismond, of Brentwood, and fled the scene.
It happened on Montauk Highway near Pease Lane, and Dorismond was pronounced dead at the scene.
Her mother, Marie Dorismond, spoke exclusively to Eyewitness News.
"My heart can't take it," she said. "Who just leave her in the middle of the road, like an animal?"
Tracey Dorismond lived with her parents, who say none of this seems real, that their youngest child, a medical assistant, is dead.
"She's been working since she was 16 years old," dad Jean Dorismond said. "A very good person."
The investigation is continuing, but it is not believed that the incidents are connected.
Both families are begging the suspects to surrender.
"If it were your child, your family, do you want to do that?" Marie Dorismond said. "That's the only thing I can say right now. My heart can't take it."
Detectives are asking anyone with information on these crashes to call the Major Case Unit at 631-852-6555 or Crime Stoppers at 1-800-220-TIPS. All calls will be kept confidential.
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