"We are going to close this case, there's no doubt in my mind," said McCarthy.
Jonathan Watkins has an extensive criminal record, and McCarthy has said Watkins belongs to a gang. He is cooperating with police in the investigation into last week's attack, the police superintendent said.
"At this point there's a lot more he can help us with, but (he) is cooperating with the investigation," McCarthy said.
McCarthy has said police have surveillance video that shows the van they believe was the getaway vehicle. And he said investigators were trying to verify the existence of an alleged Facebook post threatening Watkins.
Services for Jonylah were to be held Tuesday at a church in the Woodlawn neighborhood on Chicago's South Side where the shooting occurred. A local funeral home is paying for the services.