CHICAGO (CBS) — A woman recently got an alert on her phone about a shooting just steps outside her door in the South Shore community, and even saw the police activity out her window.

Hours later, she discovered the victim was her own brother – and a mentor in the community.

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Oliver William McCall Jr. was simply walking when he became caught in the path of rapid gunfire. Late Thursday, police were trying to determine if an armed trio who crashed a stolen car was spooked when they opened fire on the off-duty security guard.

Meanwhile, his family is searching for answers.

“We are broken,” Natasha McCall told CBS 2’s Jermont Terry.

She could not hold the tears as reality set in. Her brother, Oliver, is no longer here.

“It’s such a tremendous loss on every level, because he was a good man,” Natasha McCall said.

Last Friday morning, Oliver McCall Jr.’s life ended when someone shot him multiple times while in an alley off 73rd Street and Yates Boulevard. It was just after 6 a.m., and he was out making sure his mom got to work safely.

“He walked her to the bus stop to go to work, but he just did an overnight shift,” Natasha said.

McCall worked a security job, but did not make it back home.

“He would have done anything for a stranger on the street. That’s the truth,” his sister said, “and as a nurse, my first instincts would’ve been to breathe life into anybody I would have seen. And for someone to take his life – with such malice.”

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McCall’s shooting happened within seconds of three people crashing and running out of a stolen car down the alley. A neighborhood app alerted Natasha of the police activity.

“I actually took a picture of the crime scene,” she said.

But when she snapped an image of the bullet shell casings on her way to work, she had no idea.

“I didn’t know it was Oliver,” Natasha said.

Not until that night, when officers came to her door, did Natasha find out it was her baby brother in that alley.

“They had to have the wrong door, because we don’t put negativity out for this to come to our door,” she said.

As police search for Oliver McCall’s killer, his 4-year-old daughter, Kennedy, keeps watching the door for daddy.

“How do you tell a 4-year-old? She thinks he’s coming back,” Natasha said. “It’s going to hit her when he doesn’t show up, because he always shows up.”

It is believed Oliver McCall, who was licensed to carry a gun, returned fire at whoever shot him. It was not clear whether he hit anyone.

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The family has set up a GoFundMe to help with burial expenses and with living expenses for his daughter.