The gunfire erupted on Vassar Avenue in Newark just before 2 a.m.
Two people, identified as 25-year-old Fuquan Davis and 24-year-old Jalil Henry, were pronounced dead at the scene.
The victims were brothers.
A third person, who has not yet been identified, was taken to University Hospital in Newark in unknown condition. A fourth passenger was not injured.
Family members say the trio had gone out Thursday night to celebrate Jalil's birthday.
"He lived with me, and he always go out," grandmother Mardrella Davis said. "And I gave him the key to come back in, but he didn't come back in. And then my daughter came and told me that somebody had shot him and his brother. That they shot them and they was dead."
The investigation was focused on the vehicle, particularly one of the back passenger-side doors. The car was parked in front of the home of another relative.
Authorities say whoever approached from behind appears to have fired the shots from outside the car.
Police closed Vassar Avenue for the investigation.
