CHICAGO (CBS) – What was the past connection between a Chicago police officer and a man he allegedly shot and killed on Monday?

That is one thing investigators are looking into, after Monday morning’s shooting on Chicago’s Northwest Side.

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CBS 2’s Dana Kozlov is at police headquarters with the latest details in the case.

The victim’s girlfriend may have witnessed Monday morning’s shooting. Police were still questioning her Monday evening.

A department spokesman said internal affairs is also trying to determine if the officer had pointed his weapon at the victim prior to Monday.

“He had no weapon!”

Angelica Nieves is not only distraught. She is outraged.

Police confirm her brother, 38-year-old Jose Nieves, was unarmed Monday when an off-duty police transit officer shot him outside his apartment on the 2500 block of North Lowell.

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“He wasn’t doing anything wrong,” Nieves said. “He was moving in furniture. Literally moving in furniture.”

Chicago Police Superintendent Eddie Johnson said this was not the first time the two men came face to face.

“These two individuals had a confrontation two weeks ago,” Johnson said.

Nieves’ sister said the same officer pulled a gun on him in the past. A Chicago police department spokesperson told CBS 2 on Dec. 11, police were called to Nieves home for an alleged assault he was committing. At the time, Nieves told police, a man, who identified himself as a Chicago police officer, pointed a gun at him. Internal Affairs is now trying to determine if that officer was the same one who pulled the trigger Monday.

“If a person does not have anything on them and you’re shooting them multiple times at their body,” Nieves said. “What kinds of person are you? You’re a horrible person. You shouldn’t even be a cop then.”

The police spokesperson could not say if Nieves was charged with any crime after that Dec. 11 police call.

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His family members also believe the confrontation may have had something to do with one of the officer’s family members, but police cannot confirm that.