Chicago police late Sunday were investigating a shooting that left a man dead and two other people wounded in the South Side's Chatham neighborhood.

The shooting happened Saturday just before midnight at an unofficial high school alumni gathering in a shopping center parking lot in the 100 block of West 87th Street. The shopping center is located just off the Dan Ryan Expressway and near the 87th Street CTA Red Line stop.

Hours later, shoes and alcoholic beverage containers were left behind in the parking lot where the shooting happened. Police are now investigating the shooting as a homicide.

A quick citizen app video briefly shows people running as gunshots rang out at the scene.

"It's sickening," said Nachelle Adams, who graduated from Simeon Career Academy.

On Saturday, Adams attended the Simeon Alumni Association's 18th Annual Family & Friends Scholarship Cookout in south suburban Calumet City. She said another gathering in the shopping center parking lot at 87th Street and Lafayette Avenue, miles away from the cookout but fairly close to the school, was not an official alumni event.

"We would never walk up to, you know, anybody — or our Wolverines for sure — and start just shooting," said Adams.

Adams said she left the parking lot gathering because of the people she didn't recognize.

"When I pulled up, I saw pockets of Simeon people, and then so many jeans and black, pink and blue, and whatever like that," she said. "I was like, it's too many outsiders."

Police say the victims were all standing outside when an unknown person shot at them.

A 25-year-old woman was shot in the head and was taken to the University of Chicago Medical Center in critical condition. An aunt identified the woman as Arneka McReynolds.

A 24-year-old man was hit in the left shoulder and was taken in good condition to OSF Little Company of Mary Medical Center in Evergreen Park.

Another 24-year-old man suffered multiple gunshot wounds to the body and was taken to the University of Chicago Medical Center, where he was pronounced dead.

McReynolds' aunt says she graduated from Simeon and recently graduated from college.

"Our association, we were not aware of these various parking lot events that was occurring in instances last night," said Simeon Alumni Association public relations co-chair Shani Poston.

The alumni association posted to Facebook on Friday saying, "No gatherings or meetups using the SAA name are authorized at any other location."

"Other people, for whatever reason, have become aware of it — and they think it's like this big party, and they want to be a part of it, and they're turning it into a really big event," said Poston, "and it's just really getting out of control."

Police said the search continued for the shooter late Sunday. No one had been arrested, and detectives were investigating.