Battalion Chief Joe Ostrander with the Tri-State Fire Protection District said the shooting took place about 12:30 a.m. on Route 83 near Honeysuckle Rose Lane. A large group was gathered for a Juneteenth celebration in a strip mall parking lot that turned violent, witnesses said.
Fire officials said at least 10 were shot.
A DuPage County Sheriff's Office official said later Sunday morning that 20 had been shot, one fatally.
DuPage County Sheriff's Office update on Willowbrook shooting
"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 said.
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Two of those shot are in critical condition, while others suffered what fire officials are calling "graze wounds."
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"We were all just out, and, next thing you know shots just kept going off; everybody ran, and it was chaos," another witness said.
The victims were taken to hospitals in DuPage, Cook and Will counties.
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"I saw that they had cops laid out at all the entrances like that; I just got back home. Now they got the place all taped up," witness Kyria said.
A massive police presence remained on the scene investigating about 8 a.m., and debris could be seen covering the ground after the incident.
"We just heard gunshots; at first it was one, but then it was a whole bunch of gunshots. After then we started running like, 'yo.' I looked at my friend, and we took off, running like crazy," witness Zaviar Sheikh said.
The sheriff's office asked that anyone with information call 630-407-2400.