A gunman repeatedly shot an emergency room doctor outside a Chicago hospital, including after she fell to the ground, before he ran into the building and fatally shot a responding police officer and a pharmacy worker as she walked out of an elevator, according to witnesses and investigators.The attacker, Juan Lopez, also died during the shooting on Monday at Mercy Hospital on the city's South Side, but it was unclear if he took his own life or was killed by police, Chicago Police Superintendent Eddie Johnson said late on Monday outside another hospital, just minutes after leaving the slain officer's family."This officer, all of those officers are heroes, they saved a lot of lives because we just don't know how much damage he was prepared to do," Johnson said.Mayor Rahm Emanuel, fighting back tears, said the victims were "all going about their day, all doing what they loved". He added: "This just tears at the soul of our city. It is the face and a consequence of evil."The hospital identified the slain staff members as 38-year-old Tamara O'Neal, an emergency room physician who never worked on Sunday because of her religious faith, and Dayna Less, 25, a first-year pharmacy resident who had recently graduated from Purdue University.The slain officer was identified as Samuel Jimenez, 28, who joined the department in February 2017 and had recently completed his probationary period, Johnson said. Police said he was married and the father of three children.Police spokesperson Anthony Guglielmi described the shooting as a "domestic-related active shooter incident", but he provided no details about the relationship between the gunman and the slain doctor.Lopez, 32, and O'Neal had been arguing in the hospital parking lot just before the shooting. When one of O'Neal's friends tried to intervene, "the offender lifted up his shirt and displayed a handgun", Johnson said.The friend ran into the hospital to call for help, and the gunfire began seconds later. After O'Neal fell to the ground, Lopez "stood over her and shot her three more times", a witness named James Gray told reporters.When officers arrived, the suspect fired at their squad car then ran inside the hospital. The police gave chase.Fifteen minutes 'seemed like an eternity'Inside the medical centre, Lopez exchanged fire with officers and "shot a poor woman who just came off the elevator" before he was killed, Johnson said, referring to Less, the pharmaceutical assistant.Jennifer Eldridge was working in a hospital pharmacy when she heard three or four shots that seemed to come from outside. Within seconds, she barricaded the door, as called for in the building's active shooter drills. Then there were six or seven more shots that sounded much closer, just outside the door."I could tell he was now inside the lobby. There was screaming," she recalled.The door jiggled, which Eldridge believed was the shooter trying to get in. Some 15 minutes later, she estimated, a SWAT team officer knocked at the door, came inside and led her away. She looked down and saw blood on the floor but no bodies."It may have been 15 minutes, but it seemed like an eternity," she said.Maria Correa hid under a desk, clutching her 4-month-old son, Angel, while the violence unfolded. Correa was in the waiting area of the hospital for her mother-in-law's doctor appointment when a hospital employee told them to lock themselves in offices.She lost track of how many shots she heard while under the desk "trying to protect her son" for 10 to 15 minutes."They were the worst minutes of our lives," Correa said.The death of Jimenez comes nine months after another member of the Chicago Police Department, Cmdr. Paul Bauer, was fatally shot while pursuing a suspect in the Loop business district.Mercy has a rich history as the city's first chartered hospital. It began in 1852, when the Sisters of Mercy religious group converted a rooming house. During the Civil War, the hospital treated both Union soldiers and Confederate prisoners of war, according to its website.