A police officer and two members of staff have been killed in a shooting at a hospital in Chicago.
The attacker also died on Monday but it was not clear if he took his own life or was killed by police in a shootout at Mercy Hospital in the south of the city.
The officer who died was identified as Samuel Jimenez, a 28-year-old married father-of-three.
The hospital staff were Tamara O’Neal, 38, an emergency room physician and Dayna Less, a 25-year-old first year pharmacy resident who had recently graduated from Purdue University.
The identity of the gunman had not yet been released.
Fighting back tears, Mayor Rahm Emanuel said: “This just tears at the soul of our city. It is the face and a consequence of evil.”
Police said the tragedy was sparked by an argument in the hospital car park between the gunman and Dr O’Neal, who had a relationship with him.
When a friend of Dr O’Neal tried to intervene, “the offender lifted up his shirt and displayed a handgun,” Superintendent Eddie Johnson said.
The friend ran into the hospital to call for help and the gunfire began seconds later, with the attacker killing Dr O’Neal.
After Dr O’Neal fell to the ground, the gunman “stood over her and shot her three more times,” a witness told reporters.
When officers arrived, the suspect fired at their squad car and then ran inside the hospital.
Inside the medical centre, the gunman exchanged fire with officers and “shot a poor woman who just came off the elevator” before he was killed, Mr Johnson said, referring to pharmaceutical assistant Ms Less.
Jennifer Eldridge was working in a hospital pharmacy when she heard shots that seemed to come from outside. She barricaded the door before hearing more shots that sounded much closer.
“I could tell he was now inside the lobby. There was screaming,” she said.
The door jiggled, which Ms Eldridge believed was the shooter trying to get in. Around 15 minutes later a Swat team officer knocked at the door, came inside and led her away.
Dr Patrick Connor from Mercy Hospital described Dr O'Neal as a "wonderful individual" who was "dedicated to caring for her community".
A guard of honour was formed by hundreds of officers as Mr Jimenez's body was transported to a mortuary in an ambulance.