At least 15 people were shot, and two of them killed, at a "pop-up party" at a business on the South Side of Chicago on early Sunday morning.

The shooting happened at 4:40 a.m. A 31-year-old woman, who was shot three times including once in the head, was found dead outside the building; and a 39-year-old man, with gunshot wounds to his neck and leg, was later found dead inside a backroom in the business that operates as a tow company but also serves as an event space for pop-up parties, Chicago Tribune reported on Sunday.

The people, who were shot, are aged from 20 to 44.

Chicago police believe the shooting was gang-related. It wasn't clear how many people opened fire, the police said.

"Four guns were recovered at the scene. Our detectives are continuing to do their preliminary investigation and their interviews but several of the witnesses who were wounded are still in surgery and have yet to be interviewed, so more to come as far as any kind of motive or any additional evidence," Chicago police Superintendent David Brown said at a press conference Sunday morning. "We are following up on several different leads that we are yet unable to confirm."

According to the preliminary police report, one of those weapons was found outside the business and three others were found inside.The report said several shell casings and fired bullets were recovered from inside.

Chicago Fire Department District Chief Juan Hernandez expected the total number of people shot could grow, as some people who may have left the business show up at area hospitals on their own later in the day.

No arrests had been made Sunday morning, the local newspaper said.