Marcellis Stinnette, 19, died from gunshot wounds, according to a preliminary autopsy report released Thursday.
"We have been in close contact with Mr Stinnette's family and our deepest condolences go out to them during this difficult time," the coroner, Howard Cooper, said in a statement.
"This is truly a tragedy."
Tafara Williams, who was in the driver's seat of the car, was taken to hospital for treatment. She suffered serious injuries but is expected to recover, the coroner added.
Waukegan Police Department commander Edgar Navarro told reporters on Wednesday that an officer approached a "suspicious" vehicle to investigate it. When the officer approached, the driver of the car sped off, according to Commander Navarro.
He said the vehicle was then spotted by a second officer who got out of his car and approached.
"That officer exited his vehicle, and the vehicle that he was investigating began to reverse towards the officer. The officer then pulled out his duty weapon and fired into the vehicle," commander Navarro said.
Both Stinnette and Ms Williams, a mother of two in her 20s, were struck by bullets.
Commander Navarro said they were no weapons found in the car and did not elaborate on why the first officer had stopped the car.
He said the officer who fired the shots is Hispanic and has been with the force for five years. He was in "fear for his safety," according to Commander Navarro. The officer who first approached the car is white, police said.
Both officers have been placed on leave while authorities investigate. The Illinois State Police Department will lead the investigation into the shooting, according to Waukegan police.
The Waukegan Police Department has provided access to all reports related to the event and handed in bodycam and dashcam footage.
Commander Navarro did not put a timeline on when this footage might be released.
Ms Williams was visiting her mother Cliftina Johnson in hospital on Wednesday. "When I got there, she said, 'Mama, they just shot us for nothing,'" she told reporters at the hospital.
"My daughter said she put her hand up, and if she didn’t put her hand up, she said, 'Mama, I would be dead.'"
Protesters took to the streets in Waukegan on Thursday to demand justice for Stinette and Ms Williams.
"Say his name! Marcellis Stinnette! What's her name? Tafara Williams," the group chanted as they marched through the small city which has a population of around 86,000.
According to the latest census data, the city is majority Hispanic or Latino and around a quarter black.
“They were not thugs. They were not problematic kids,” Ricky Carly, a family spokesman, told NBC Chicago. “They didn’t have any issues with anybody out here.”
“We’re looking for peace and justice. That’s all we want,” Francellise Stinnette-Watts, the great aunt of Marcellis, said. “We want to know what happened. We would like to see the video cam.”