Flags outside a Chicago police station were lowered to half staff Tuesday in honor of the fallen police officer, identified as 28-year-old Samuel Jimenez. Mercy Hospital identified the staff members who died as 38-year-old Tamara O'Neal, an emergency room physician who never worked on Sunday because of her religious faith, and 25-year-old Dayna Less, a first-year pharmacy resident who had recently graduated from Purdue University.
The attacker, Juan Lopez, also died Monday but it was not clear if he took his own life or was killed by police at Mercy Hospital on the city's South Side, Chicago Police Superintendent Eddie Johnson said.
Related: What we know about Mercy Hospital shooting suspect Juan Lopez
The 28-year-old officer who ran into the line of fire to save others worked in the city's 2nd Police District and joined the department in 2017. He is being remembered as a loving husband and father of three young children.
Officers stood at attention Monday night as an ambulance carrying his body passed in a procession to the medical examiner's office.
Police were called to Mercy Hospital around 3:30 p.m. after authorities say the gunman confronted O'Neal, his ex-fiancee. The couple's engagement was called off in September.
Lopez, 32, and O'Neal had been arguing in the hospital parking lot. When a friend of O'Neal 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.
Lopez then shot and killed Less as she was coming off of an elevator, according to authorities, before exchanging gunfire with Officer Jimenez and other officers. Jimenez was shot in the lobby, while a second officer had a bullet hit his holster.
"Those officers who responded today saved a lot of lives because this guy was shooting that poor woman who had got off an elevator," Johnson said. "Had nothing to do with nothing, and he shot her. There's no doubt in my mind that all those officers who responded saved a lot of lives because we just don't know how much damage he was prepared to do."
The suspect was found inside the hospital with a gunshot wound to the head.
The Chicago Police Memorial Foundation said that they will present Officer Jimenez's family with a check for $60,000 to help with expenses. The foundation also said they will take care of any education related expenses for his children.
As the shooting took place, several of the victims' colleagues and patients took cover, hiding where they could as the gunman made his way inside. As bullets flew in the lobby, pharmacy workers huddled behind a locked door.
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," she said. "There was screaming."
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.
(The Associated Press contributed to this report)
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