By Leah Asmelash, Kay Jones, Brad Parks and Ryan Young, CNN

(CNN) -- Almost two years after an Illinois police officer fatally shot a Black man working as a security guard, the state's attorney's office announced no criminal charges will be filed against the officer.

"After an extensive and thorough review of the police-involved shooting resulting in the 2018 death of Jemel Roberson, the Cook County State's Attorney's Office (CCSAO) has concluded that the totality of the evidence is insufficient to support criminal charges against Midlothian Police Officer Ian Covey," the news release from the attorney's office states.

Beatrice Roberson, Jemel Roberson's mother, said she was "devastated" by the decision and that State's Attorney Kim Foxx let her family down by not bringing charges against Covey.

"I will not stop fighting for Jemel as long as I live," she said at a news conference. "As long as I live, I will be trying to get justice that he deserves. My family and I will never be able to put this behind us."

The decision comes after months-long protests against police brutality, particularly against Black people, and just over a week after none of the three officers involved in Breonna Taylor's shooting were charged with causing her death.

"I am acutely aware in this age of civil unrest that police-involved shootings are viewed under a microscope, as they very well should be," Foxx said in a statement. "The death of Jemel Roberson is tragically heartbreaking, and while it might feel to some people like justice was not served here, I have both an ethical and legal obligation to make charging decisions based on the law and the evidence."

The investigation included interviews with over 100 witnesses and the evaluation of physical evidence and information surrounding the event, the CCSAO said.

A secondary review of the their investigation was conducted by the Office of the Illinois State's Attorney Appellate Prosecutor, who agreed that no criminal charges should be filed against the officer, the CCSAO said.

The incident occurred in November 2018 when Roberson, 26, was working security at Manny's Blue Room, a bar in suburban Chicago. When an active shooter came in and began firing, Roberson pinned the alleged gunman to the ground, a witness said.

Moments later, Covey, a White Midlothian police officer, arrived and killed the security guard, the police chief said.

"We all yelled, 'He's a security. He's a security,' and without ... giving any thought, they shot him," Adam Harris, a witness, told CNN affiliate WGN. "The vest said security as well ... and they shot him in the side."

The shooter was not killed. It's unclear why he was shooting in the first place.

Midlothian Police Chief Daniel Delaney called the incident "tragic" at the time.

"What we have learned is Jemel Roberson was a brave man who was doing his best to end an active shooter situation at Manny's Blue Room," Delaney wrote on Facebook.