A police officer shot and killed a security guard at a suburban Chicago bar on Sunday, and now the guard's mother is suing police, after witnesses said he had subdued a gunman who opened fire at the club.

Key points: Four others were injured in the shooting

Four others were injured in the shooting Jemel Roberson, who was black, was the only person killed

Jemel Roberson, who was black, was the only person killed A civil rights lawsuit has been filed on behalf of Mr Roberson's mother

Jemel Roberson, 26, was pronounced dead shortly after being taken to a hospital following the shooting at Manny's Blue Room in Robbins, just south of Chicago.

Four others were shot and injured, including a man who police believe fired a gun before the police arrived, Cook County sheriff's spokeswoman Sophia Ansari said.

When police arrived at the scene, Mr Roberson was holding "somebody on the ground with his knee in his back, with his gun in his back," witness Adam Harris told local station WGN-TV.

"Everybody is screaming out, 'He's a security guard,'" Mr Harris said.

Mr Roberson, a father of a nine-month-old boy, was licensed to carry a firearm, Ms Ansari said.

Charges were pending against the man who investigators believe fired the initial shots during a dispute.

His name was yet to be released, and he remained hospitalised, Ms Ansari said.

Investigators said the initial gunfire was reported about 4:00am.

An officer responding from nearby Midlothian encountered and shot "a subject with a gun", Midlothian Police Chief Daniel Delaney said in a statement. The person was later identified as Mr Roberson.

No details have been released about the officer.

Loading

Mr Roberson, who was black, was the only person killed.

The other four people who were shot suffered wounds that were not considered life-threatening, Ms Ansari said.

Attorney Gregory Kulis filed a civil rights lawsuit seeking more than $US1 million on behalf of Mr Roberson's mother, Beatrice Roberson. He said Mr Roberson was dressed in black but wearing a hat with the word "security" when he was shot.

Mr Kulis also echoed witness reports that Mr Roberson was holding down another man outside the bar when the officer arrived and shot him.

The Reverend Marvin Hunter said Mr Roberson was "an upstanding young man" and a promising keyboard player at his and several other area churches.

Mr Roberson was trying to "get enough money together for a deposit on a new apartment", said Reverend Hunter, the great uncle of Laquan McDonald, a black teenager fatally shot by a white Chicago police officer in a high-profile 2014 case.

Mr Roberson also had hopes of someday becoming a police officer, according to his son's mother.

"This was going to be my baby's first Christmas with his dad and now he's going to miss out on everything," Avontea Boose said.

Illinois State Police are handling the investigation into Mr Roberson's shooting.

The agency said it would not comment on the investigation until it was completed.

Map The shooting occurred about 4:00am outside a suburban Chicago bar.

AP/ABC