CHICAGO (CBS) — A Midlothian police officer shot and killed a security guard at a south suburban bar on Sunday, and now the guard’s mother is suing police, after witnesses said the Jemel Roberson had subdued a gunman who opened fire at the club.

Family and friends are demanding answers, after a Midlothian police officer shot and killed 26-year-old Jemel Roberson outside Manny’s Blue Room Lounge, at 2911 S. Claire Blvd. in Robbins.

When witnesses said a fight broke out around 4 a.m. Sunday, and someone started shooting. Four people were wounded, including the gunman.

The shooting prompted a massive police response. Witnesses said Roberson was armed, and had tackled a suspect, when police arrived at the scene.

Midlothian police and Cook County Sheriff’s officials confirmed officers from several agencies responded to the shooting, and a Midlothian police officer shot an armed person at the scene, who turned out to be a security guard.

Witnesses and Roberson’s family and friends said police shot the wrong person.

“We’re still processing. To be on the job, saving lives – again, which is what he loves to do, just to help people – and then to lose his life, in essence doing what he wanted to do, is just complete devastation,” said pastor and family friend LeAundre Hill.

Roberson’s mother has filed a federal lawsuit against the unnamed officer and the village of Midlothian, accusing the officer of using excessive force. The lawsuit alleges the officer’s decision to shoot Roberson was unreasonable, unprovoked, and unjustified.

Witnesses said Roberson was armed, holding down the gunman responsible for the initial shooting, when a Midlothian police officer shot him outside the bar.

Adam Harris said he was inside Manny’s when the initial shooting happened.

“I ducked down in the bar. Then, when it got calm, and we went out, we saw the security guard got killed last night. He had somebody on the ground, with his knee in his back, with his gun on him, like ‘Don’t move,’” Harris said. “Everybody was screaming out ‘Security!’ He was a security guard.”

Investigators have not said whether the officer gave Roberson any verbal commands.

One woman who saw what happened said the officer was running out from the bar. She asked CBS 2 not to show her face on camera.

“You hear the crowd yelling and screaming, but when he comes out of that door with that gun up, it’s just pop pop pop pop pop pop pop pop,” she said. “The other officers were out here. They know him. They seen him, and once the police actually shot him, everybody started screaming. The officers started screaming, ‘He’s one of us. He’s security.’”

Roberson was pronounced dead at Advocate Christ Medical Center in Oak Lawn.

Hill described Roberson as a hard worker, and a pianist who loved to play gospel music. Roberson also had aspirations of becoming a police officer.

Hill said Roberson’s family just wants to know how and why this could happen.

“They just want answers, so that his mom, his loved ones can have some peace, if there’s ever any peace that comes with these type of situations.”

As is typical for a police shooting in the suburbs, the Illinois State Police Public Integrity Task Force was investigating the shooting.

Midlothian Police declined to comment on the circumstances of the shooting, until the state police investigation is completed.

“The officer, per standard protocol, is on paid administrative leave during this investigation,” Midlothian Police Chief Daniel Delaney stated in an email.

Delaney said the officer involved has been with the Midlothian Police Department for about four years, and before that was an officer at another department for three years. The officer is a Midlothian patrol officer, and team leader for the South Suburban Emergency Response Team, a regional SWAT team.