At least 15 people were shot and wounded, two of them fatally, at a "pop-up" party in a Chicago garage in the early hours on Sunday, police and local media reported.
The event was held at a tow company premise converted for parties on the city's South Side, police said.
There was a disturbance "among several patrons and gunfire erupted striking multiple people," police officer Jose Jara said in an email to the Chicago Tribune newspaper, adding the cause of the shoot-out was not known.
He said the people shot were aged between 20 and 44. Four weapons were found at the scene, and several witnesses were still in surgery later Sunday.
The city's fire service confirmed a provisional toll of two dead.