Mayor Brandon Johnson said it was known that teenagers were planning to flood the downtown area on Friday night.
Hundreds of extra police officers were assigned, he said.
Still, there were two shootings that left 8 teens wounded and another dead.
The incidents occurred outside the Chicago Theater and at Dearborn and Adams.
Video at the theater scene showed people running for cover and police flooding the area.
Seven teens were shot there.
They’re between 13 and 17 years old.
None had life-threatening wounds.
The 14-year-old boy who died following the shooting at the second scene was shot multiple times.
An 18-year-old was wounded there.
Mayor Brandon Johnson said police were aware young people were organizing a large gathering.
“We had been monitoring social media, this gathering for a couple of days,” he said. “And so working with the Chicago Police Department and the Chicago Public Schools, there was a communication that was delivered to CPS to encourage young people not to participate in this unauthorized gathering.”
He said there were an additional 700 police officers assigned to the downtown area. “But clearly what we put in place did not do enough to prevent what we were concerned about from actually manifesting.”
The mayor said “incidents like last night set us back as a city.”
He added, “I’m the first person to recognize that we have more work to do in this city.”