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This is a representational image showing police at a murder scene on June 30, 2017 in Chicago, Illinois. Getty Images/Scott Olson

Jemel Roberson, the 26-year-old security guard of a bar who was fatally shot by a suburban Chicago police officer responding to a shooting Sunday, was “a brave man who was doing his best to end an active shooter situation,” according to protesters. The fatal shooting of Roberson sparked outrage as the officer shot him despite bystanders saying that he was just a security guard and not the suspect.

An attorney for the slain man's family said Tuesday that Roberson was just "doing his job" when he was shot. The incident took place at Manny's Blue Room Lounge in Robbins, about 20 miles south of Chicago. Police arrived at the scene after learning of an active shooter incident. Three people and an alleged shooter were injured, Cook County Sheriff's Office spokeswoman Sophia Ansari told CNN.

Roberson had apprehended an alleged gunman at the scene but the Midlothian officer shot him outside the bar, according to a federal lawsuit filed Monday. The fatal shooting has been called "excessive and unreasonable" and it violated Roberson's civil rights, according to the suit, which is seeking justice for Roberson.

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

A statement posted by Midlothian Chief Daniel Delaney on Tuesday said the department was “completely saddened by this tragic incident” and offered “heartfelt condolences” to the security guard's friends and family.

"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.

The Midlothian Police Department also asked the Illinois State Police Public Integrity Task Force to investigate the incident as it is the department policy for an officer-involved shooting, Delaney said.

In a statement, the Illinois State Police said, "the Midlothian Officer gave the armed subject multiple verbal commands to drop the gun and get on the ground before ultimately discharging his weapon and striking the subject."

The statement also dismissed witness claims saying Roberson was in plain clothing with no markings that identified him as a security guard of the bar and armed with a gun.

The suspected gunman in the initial bar shooting incident is being treated at a local hospital and has not been charged, Ansari said.

A GoFundMe page has been set up for Roberson to help pay for funeral costs. "Jemel Roberson was a young man who was greatly loved by all who knew him. He was the light of his mother Beatrice's life, and was a devoted loving son. Jemel was a gifted basketball player and musician, and his love for God and his family were at the forefront of his life. Sadly, Jemel's life was tragically and unexpectedly cut short this morning as he tried to save others from senseless violence. Today, many remember him as a hero; one who put others before himself," the description on the site read.