Police believe an assault-style rifle was used in the gang shooting that injured 13 people, including a three-year-old boy.
No suspects are currently in custody.
CNN's Deb Feyerick is OutFront in Chicago.
Amanda Knox, accused of murdering her roommate while studying abroad - now faces retrial in Italy later this month.
Her acquittal was overturned by the Italian courts earlier this year.
Knox told NBC's Today Show that she has no plans to go back.
"I was already imprisoned as an innocent person in Italy, and I can't reconcile the choice to go back with that experience. It's not a possibility. I was imprisoned as an innocent person, and I just can't relive that," Knox said.
But she isn't the one who will decide if she goes back. The United States government will. Should it extradite her back to Italy?
CNN Legal Analyst Paul Callan is OutFront.
The U.S. government is doing a whole lot of outsourcing and it may be putting lives at risk.
In fact, one of the biggest private government contractors conducted the background check for Edward Snowden, whose leaks lawmakers say cost lives. The same company, USIS, also vetted the navy yard shooter.
Are they to blame or is Uncle Sam still holding the buck?
Pentagon correspondent Chris Lawrence is Outfront with this investigation.