At least 20 people were shot, one fatally, early Sunday in a strip mall parking lot in unincorporated Willowbrook, officials said, in a gathering witnesses said was a Juneteenth celebration.

The shooting happened shortly before 12:30 a.m. as a group of people gathered in a parking lot near Route 83 and Honeysuckle Rose Lane, DuPage County Sheriff Deputy Chief Eric Swanson told reporters Sunday morning.

Sheriff deputies in the area heard the gunfire and responded, according to Swanson, who said “at least 20” people were wounded by gunfire.

The victims were taken to hospitals in DuPage, Will and Cook counties, he said. Several of the victims took themselves to hospitals, he said.

It was unclear what prompted the shooting or how many shooters were being sought, Swanson said. He did not disclose the age or genders of any of the victims.

One witness told ABC7-Chicago people were gathered for a Juneteenth celebration when gunfire rang out.

“It was supposed to be like a Juneteenth celebration; we just started hearing shooting, so we dropped down until they stopped. They just kept going. After that, we literally scattered away,” witness Markeshia Avery told the TV news station.

The shooting is being investigated by the DuPage County sheriff’s office and Metropolitan Emergency Response and Investigative Team, Swanson said.

He asked anyone with information to call 630-407-2400.