A gunman on a Chicago subway platform opened fire at an arriving train in a targeted attack, police say.
A male victim was shot and was taken to a nearby hospital, where he is in a stable condition.
Chicago police deputy Superintendent Kevin Navarro said the shooter waited for the victim at the Chicago Transit Authority's Jackson station, which is in the city's downtown business district.
Mr Navarro said: "As the doors opened up, he shot at the specific subject and ran."
The suspect was not in police custody by Friday morning.
Mr Navarro said it was a startling but rare event. He said there was no general threat to the public.
Detectives are reviewing surveillance footage and extra subway patrols are planned.
AP