The US city is notorious for gun crime, but Tyshawn Lee's November 2 death managed to horrify a Chicago already reeling from more than 450 murders this year.
"This is a crime that shook our city," city police superintendant Garry Mccarthy told reporters.
"It was an act of barbarism: The assassination of a nine-year-old child as a gang retaliation to get back at his father."
McCarthy said 27-year-old Corey Morgan had been arrested and charged and cited the suspect's criminal history as an illustration of the challenge facing Chicago.
"When we solve this case," he said, "we're going to find an individual with an extensive, violent criminal history who likely should not have been on the streets.
"That's the way it happens almost every single time. Corey Morgan is a documented gang member, a convicted felon," he complained.
"As you know, we picked him up just a couple weeks ago with yet another firearm. So he has a pending gun arrest along with a history of violence."
Another suspect has been arrested and a third is being sought, he added, predicting that further charges would be brought in the case.
'Targeted and executed'
The investigating officer, Sergeant William Svilar, said that, while residents were terrified of retaliation, the local community had rallied to help detectives.
"It's a nine-year-old boy targeted and executed," he said.
"People understand that when you have a bad element in your neighborhood, you don't run from the neighborhood. You get rid of that element," he said.
"But there was a lot of fear, which is completely understandable.
"If you have a monster who's willing to assassinate a nine-year-old, what is that person likely to do if they know somebody is cooperating with the case?"
Police would not be drawn in detail as to what Lee's father -- an alleged gang member -- had done to provoke the men who retaliated by murdering his son.
"There's a number of events that precipitated this particular murder. A couple of them are murders. A couple of them are non fatal shootings," Svilar said.
Lee's body was found riddled with bullets not far from his home and a park in which he had been playing basketball with friends on Chicago's troubled South Side.
South Side has seen a drastic rise in homicides in recent months, but Lee's death in particular has prickled the conscience of many in Chicago.
Mayor Rahm Emanuel vowed earlier this month to find and prosecute Lee's killers.
"Whoever did this, there is a special place for them," he said. "I hope they never see daylight."
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