Tafara Williams' boyfriend, Marcellis Stinnette, was also shot. He died from his injuries.
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Williams' family and attorneys gathered outside of Waukegan's City Hall while she appeared virtually.
"He's dead at the hands of the people who are supposed to protect us," said Dhanellis Banks, Stinnette's sister. "It's not supposed to happen."
"This officer should undergo a very serious criminal investigation as to why he shot," said Antonio Romanucci, family attorney.
A vigil was also held Tuesday at the location Stinnette was killed. Rev. Al Sharpton, a civil rights leader, was among the people who attended the vigil.
"We cannot in any way, shape or form stop until there is criminal accountability," Sharpton said.
Williams' family members call for justice
Family members of Tafara Williams call for justice
Williams said she was in a parked car to smoke outside while Stinnette was in the passenger seat when the officer pulled up from behind without his lights or sirens on.
Williams said she turned on the lights inside her car and rolled down the windows. She said the officer then called Stinnette by name and began harassing him.
'He stood near the car with his left hand on his gun and he said to Marcellus, 'I know you from jail,'" Williams said.
The officer then stepped away and Williams said she drove away slowly because she was scared.
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"I drove out to MLK and turned onto it. The officer was not following me. The police lights were not on. When I turned onto MLK, it seemed like there was another officer there waiting for us. There was a crash and I lost control. The officer was shooting at us. The car ended up slamming into a building. I kept screaming 'I didn't have a gun,' but he kept shooting."
The officer who shot the couple said he opened fire on their car as it drove toward him in reverse.
Williams said she was dragged away from Stinnette and said they covered him while he was still breathing.
"I lost the love of my life and the father of my seven-month-old son," Tafarra Williams.
WATCH: Statement from Waukegan police commander
Waukegan police officer shoots teen following traffic stop: police
Authorities said it all started with a report of a suspicious vehicle near Liberty and Oak just before midnight on Tuesday, but families said the couple was simply sitting inside their car outside her mother's home. Police said when an officer went to investigate, the car drove off.
Moments later, another officer spotted the car near Martin Luther King Jr. Avenue and South Avenue, police said. As that officer approached on foot, the car reversed and the officer fired his pistol out of "fear for his safety," police said.
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Tafarra Williams and the family of Marcellis Stinnette speak after the fatal incident.
"A second officer located the vehicle in the area of MLK and South Street," said Waukegan Police Commander Edgar Navarro. "The officer exited the vehicle and the vehicle that he was investigating then began to reverse. The officer fired into the vehicle."
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At the news conference Tuesday, Williams' family members called for justice.
"This happened to my child and her boyfriend this time, but if America doesn't stop this disease of violence, this could be anyone's child. I am demanding justice for Marcellus and Tafara Williams," said Tina Johnson, Tafara's mother.
"J-U-S-T-I-C-E. That's all we're asking for. Is that too much to ask for?," asked Eshante Williams, Tafara's brother.
While the family demands answers and change, Marcell's' mother, Zharvelis Stinnette Holmes, wants it done the right way.
"Please don't tear up Waukegan. Please, no fighting. We got to do this the right way," Stinnette Holmes said.
Williams is being represented by attorneys Ben Crump and Antonio Romanucci, who said they have met with the Mayor of Waukegan, Sam Cunningham, who has promised transparency. The name of the officer has not been released yet.
The families are still waiting to see the video from police body cameras. Police said they would show the families the video before releasing it to the public. The families hoped it would happen Tuesday, but it will likely happen by the end of the week now, families say. In the meantime, they are calling for the officer to face criminal charges.
Williams' attorneys said even though they applaud the quick termination of the officer, they still plan to file a lawsuit.
"The fact that they terminated the officer when they saw the video and saw wrong," said Crump.
Waukegan Mayor Sam Cunningham said he plans to release video of the shooting after the families see it.
The FBI and Illinois State Police are investigating. Lake County State's Attorney Michael Nerheim said he asked the U.S. Justice Department to review the circumstances surrounding the shooting.
Another march is planned for Wednesday afternoon at the scene of the shooting.