Fred Waller, who heads Chicago's police department's patrol bureau, said the shots were first fired inside a residence stemming from a dispute early Sunday morning.
When people started to escape to the streets, someone continued shooting at them from inside.
Four of the 13 wounded people are still in critical condition, Waller told reporters. The victims are between 16 and 48 years old.
Waller, who called it an "isolated incident," said they are questioning two suspects, and one of them was detained at the scene with a gun. The other was among the wounded and received treatment.
Chicago Mayor Lori Lightfoot described the incident as a "terrible tragedy" during a press conference outside a local hospital, where most of the victims were being treated.
She urged people knowing what happened inside the house to overcome fears and come forward with information.
"We can't normalize this kind of behavior and tragedy in this city," she told reporters.
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