Before she was shot and killed outside the Chicago hospital where she worked, Dr. Tamara O’Neal was an endlessly persistent physician who was devoted to making sure her patients felt cared for, her colleagues said.

Dr. O’Neal, an emergency room physician at Mercy Hospital, south of Chicago’s downtown, sometimes had trouble disconnecting herself from work when she was off duty, said Dr. Adele Cobbs, the hospital’s assistant director for the Emergency Department. Dr. O’Neal would often call during her free time just to check that her patients were healthy, she said.

“She felt very strongly about serving the underserved,” Dr. Cobbs said. “Being an emergency room physician was a way to reach the masses.”

On Monday afternoon, when Dr. O’Neal left the hospital, she encountered a man who family and colleagues identified as her former fiancé, and the two got into an argument. When someone in the hospital parking lot intervened, the man lifted his shirt and showed a handgun, the police said. Then he opened fire, they said, killing Dr. O’Neal.