Security guard fatally shot by officer 'was doing his best to stop an active shooter': Police chief Jemel Roberson's family has filed a wrongful-death lawsuit against the officer.

The Illinois police officer who fatally shot an armed security guard who was trying to stop an active shooter in a nightclub has been placed on administrative leave, authorities said on Tuesday, according to ABC News affiliate WLS-TV in Chicago.

A little after 4 a.m. Sunday, police in Robbins, Illinois called for assistance from additional area police departments to assist its officers in responding to a call about a shooting at Manny's Blue Room Bar, according to Midlothian police.

Midlothian police said two of its officers had responded to the bar to help Robbins police. When officers arrived, Midlothian police said, they learned that there were several gunshot victims inside the bar.

"A Midlothian officer encountered a subject with a gun and was involved in an officer-involved shooting," Midlothian Police Chief Daniel Delaney said in a statement released hours after the shooting.

The person shot by the officer was later pronounced dead at a hospital, Delaney said.

Midlothian police and the Cook County Medical Examiner's Office identified the deceased as 26-year-old Jemel Roberson.

The Midlothian police officer placed on administrative leave has not yet been publicly identified.

Delaney told ABC News on Tuesday that Roberson was the establishment's security guard.

"Our hearts go out to Jemel and his family and we are devastated by this incident," Delaney told ABC News. "We have learned a lot about Jemel over the last two days and he was [a] great man that was doing his best to stop an active shooter that evening."

At least four other people were shot and wounded, the Cook County Sheriff's Office said.

"He had somebody on the ground, with his knee in his back, with his gun on him, like 'Don't move,'" Adam Harris told a CBS affiliate. "Everybody was screaming out 'Security!' He was a security guard."

Roberson was an organist at local churches including New Spiritual Light Baptist Church, WLS-TV said.

This is a young man who was trying to do the right thing in life. He wanted to apply to the Chicago PD from what I was told. His mother did not want him working nightclub security but he enjoyed it.

"How in the world does the security guard get shot by police?" Pastor Walter Turner of New Spiritual Light Baptist Church said in an interview with ABC News affiliate WLS-TV in Chicago. "A young man that was literally just doing his job and now he's gone."

A lawyer for the Roberson family has filed a wrongful-death civil lawsuit against the as yet unidentified officer and the village of Midlothian.

"This is a young man who was trying to do the right thing in life," said Gregory Kulis, the attorney for Jemel Roberson's mother, Beatrice Roberson. "He wanted to apply to the Chicago PD from what I was told. His mother did not want him working nightclub security but he enjoyed it."

The Illinois State Police Public Integrity Task Force is investigating the police shooting, which is the Midlothian Police Department's policy, according to Delaney.

The Cook County Sheriff's Police and Robbins Police Department were investigating the "criminal aspect," Delaney said.

A rally was set for 4 p.m. ET Tuesday at the Midlothian Police Department.

ABC News' Andy Fies and Susan Schwartz contributed to the reporting in this story.