(CNN) By all accounts, Jemel Roberson courageously stopped an active shooter in a Chicago-area bar.

After gunfire erupted, the 26-year-old African-American security guard pinned the alleged gunman to the ground, a witness said.

But moments later, a white Midlothian police officer arrived and killed the security guard, the police chief said. Now, even the police chief is grieving the tragedy.

"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," Midlothian Police Chief Daniel Delaney wrote on Facebook.

"The Midlothian Police Department is completely saddened by this tragic incident and we give our heartfelt condolences to Jemel, his family and his friends. There are no words that can be expressed as to the sorrow his family is dealing with."

"We view this as the equivalent of a 'blue on blue,' friendly fire incident," the police chief said in a press release.

Security guard Jemel Roberson, 26, wanted to become a police officer.

Police responded to the shooting around 4 a.m. at Manny's Blue Room Lounge in Robbins, about 20 miles south of Chicago, Cook County Sheriff's Office spokeswoman Sophia Ansari said. Three people and an alleged shooter were injured, she added.

Then, after Roberson had apprehended the alleged gunman, the Midlothian officer shot Roberson outside the bar, according to a federal lawsuit filed Monday. The suit calls the fatal shooting "excessive and unreasonable" and says it violated Roberson's civil rights.

Witnesses said security had asked several drunk men to leave the bar, and at least one person returned and opened fire, CNN affiliate WGN reported. A bartender was among those victims, according to the lawsuit.

After that shooting, Roberson "had somebody on the ground ... with his knee in back, with his gun in his back like, 'Don't move,' " Adam Harris, a witness, told WGN.

The officer responding to the scene then fired at Roberson, the TV station reported, citing witnesses.

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

The unnamed officer who fatally shot Roberson has been a member of the Midlothian department for nearly four years, according to Delaney. The officer is assigned to the patrol division and also leads a regional swat team, the police chief said.

He also spent three years with another department, according to Delaney.

The officer is on paid administrative leave, as is standard department policy, pending the outcome an investigation by the Illinois State Police Public Integrity Task Force investigation.

Midlothian police had asked the state police task force to investigate, which is department policy for an officer-involved shooting, according to Delaney.

In a statement, the Illinois State Police said its officials are investigating the shooting. It said that according to witness statements, "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."

It also said Roberson was armed with a gun and wearing plain black clothing with no markings readily identifying him as a security guard.

The department did not initially identify Roberson as a security guard because it immediately relinquished the investigation to the State Police and did not know his name or position at the time, Delaney said Tuesday.

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

'Jemel was trying to save people's lives'

The lawsuit filed by Roberson's mother lists the unnamed Midlothian officer and the village of Midlothian as defendants. It seeks damages of more than $1 million.

"Jemel was trying to save people's lives. He was working security because a shooting had just taken place in the establishment," the family's attorney, Gregory Kulis, told CNN affiliate WLS

"So, he was doing his job and holding on to somebody until ... police arrived," Kulis said. "And a police officer, our feeling is, did not make the proper assessment and fired and killed Jemel."

Roberson had a Firearms Owners' Identification Card, which allows Illinois residents to legally possess firearms or ammunition, according to Ansari.

Karen Sheley, director of the Police Practices Project for the American Civil Liberties Union of Illinois, suggested that Roberson's race influenced the decision to shoot.

"Working as a security guard while Black should not be a death sentence. In this case, police were more dangerous to him than an active shooter who he apparently subdued," Sheley said in a statement

"The police killing of Mr. Roberson was avoidable. Here, a police officer's assumptions about Mr. Roberson, apparently impacted by bias, rather than a clear-headed assessment of the situation, resulted in a tragedy."

A church musician who wanted to be a cop

Jemel Roberson's young son is now 9 months old.

Roberson was the father to a 9-month-old son, and the boy's mother is expecting their second child, according to family attorney S. Lee Merritt.

"My baby's not going to have his father for Christmas, his birthday, any holidays anymore," Avontea Boose, mother to Roberson's children, told Don Lemon on "CNN Tonight" Wednesday.

He was a gospel musician at several churches, according to according to WGN . Kulis said Roberson played the organ and was scheduled to perform at church on the Sunday morning he was killed.

Roberson was also preparing to embark on a career as a police officer, WGN reported. Roberson had planned to take a police exam in March, according to activist Eric Russell.

He's a protector of everybody," Boose said. "To anybody he knew, he got close to, he was a protector."

"The very people that he wanted to be family with took his life," the Rev. Patricia Hill of Purposed Church said.

Roberson's family members are "just distraught," said another pastor, the Rev. Walter Turner.

"They are hurt, trying to find out answers, trying to figure out why did this have to happen," Turner told WGN.

Loved ones and supporters held a vigil Monday night outside Manny's Blue Room Lounge, WGN reported.