At least 15 medic units were sent to Mercy hospital, Chicago Fire Department Public Information Officer Larry Merritt said.
"We heard some shots so we all ran to the emergency window, we see this guy out there, military style, shooting at the police - pow, pow, pow", he said.
Police are continuing to do a "methodical search" of the hospital, according to Guglielmi.
Multiple victims were reported, but it was not immediately clear how many people had been shot. "He is in critical condition but receiving excellent care".
Reports are of an active shooter, along with multiple victims, at Mercy Hospital on 2525 S. Michigan Ave.in Chicago.
Senior Northern Ireland politicians meeting Taoiseach for Brexit talks
The Brexit deal is complex and unwieldy. "We would find ourselves after a very chaotic opening period having to agree something, having to agree something around aligning regulations and customs", he said.
Police spokesman Anthony Guglielmi said at least one potential suspect was shot but offered no details.
Witness James Gray told reporters that he saw a man and a woman who looked like they were walking towards the parking lot. I was in the hospital at mercy over the weekend and I was just about to go back an hour ago bc I still feel frightful but I told my mom and aunt I needed to take a nap first. Employees and patients at the clinic locked the doors and hid in the back.
"I don't know what happened", she said as she was ushered onto a bus.
"I am scared as hell".