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Four people were shot and two are dead, including the gunman, after gunfire erupted at Mercy Hospital in Chicago.
A police officer also suffered critical injuries in the shooting, with reports of gunfire from both inside and outside Mercy Hospital.
Some patients had to be evacuated and others were told to lock their doors as Chicago police scrambled to secure the building.
Witness Hector Aditia said he saw the gunman shoot a female victim dressed in hospital scrubs "multiple times in the parking lot" and then exchange gunfire with responding officers.
A massive police operation began after reports of an active shooter and multiple victims.
One officer has been shot and is in a critical condition.
A Twitter user posted video of what appeared to be Chicago police officers searching the interior of the hospital - and a blood trail on the hospital floor.
— Dommi (@DLeilaniG) November 19, 2018
Witness James Gray told reporters that he saw a man and a woman who looked like they were walking towards the parking lot.
"The gentleman turned around and shot three times in the chest," Gray said.
"Once she fell to the ground, he turned around and shot her three more times."
Gray described the shooting as surreal, "like a movie scene". He said the shooter and the woman had been speaking before the shooting, but he didn't know if they knew each other.
"They were walking and talking and he just turned around and started shooting. It wasn't a heated exchange. It was just like we're talking now," he said.
The gunman then began firing randomly.
"Once he entered he just started shooting at random," Gray said.
"It looked like he was turning and pointing at people at random.
""It was chaos, mass chaos."
A #ChicagoPolice officer has been shot in the active shooter incident at Mercy Hospital. He is in critical condition but receiving excellent care. Please send your prayers. pic.twitter.com/kFfMY1ZmF7
— Anthony Guglielmi (@AJGuglielmi) November 19, 2018
Nigary Thompson, who works at the hospital's family clinic, said she heard eight or nine shots. The clinic's employees and patients locked the doors and hid in the back of the clinic.
"I am scared as hell. I have never been so scared, I hear of shootings going on every day at people's workplaces, but not where I work at," she said.
"This was very too close to me. That could have been us back there and if any bullets had pierced the wall we all would have been hurt," Thompson said.
Police say "at least one possible offender is shot" and at least 10 ambulances were dispatched.
Reports of multiple victims from incident at 26th and Michigan. Please avoid area. pic.twitter.com/VtDIr5b2Ok
— Anthony Guglielmi (@AJGuglielmi) November 19, 2018
p class="element element-paragraph">Officers were doing a "methodical search" of the hospital, a spokesman said.
The shots were fired about 3.28pm local time near 26th Street and Michigan streets.
A source in the Chicago Fire Department told NBC 5 the hospital had been partially evacuated.
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