Chicago police have secured Chicago's Mercy Hospital & Medical Center on Monday following a shooting that left four dead, including two hospital employees, a Chicago police officer and the suspected gunman.

Mercy Hospital & Medical Center released a statement late Monday afternoon saying patients are now safe.

Chicago Police officers walk outside Mercy Hospital on the city's South Side where authorities say a shooting at the hospital has wounded multiple people, including a suspect and a police officer, Monday, Nov. 19, 2018, in Chicago. (Photo: AP)

Chicago Police Superintendent Eddie Johnson says the shooting began outside the hospital with a "verbal altercation" in the parking lot between people who knew each other. A friend tried to intervene and the suspected gunman pulled up his shirt and showed a weapon. Gunfire erupted and the shooter ran inside the hospital, where police confronted him.

He says that one of the women killed was in a domestic relationship with the gunman.

A witness named James Gray told reporters that it looked as if the attacker "was turning and shooting people at random."

Television footage showed several people, including some wearing white coats, walking through a parking lot with their arms up.

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, now 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 in 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 told a reporter.

Maria Correa was at the hospital for her mother-in-law's appointment with a doctor. As the violence unfolded, a hospital employee locked the doors to the waiting area. Correa hid under a piece of furniture, clutching her 4-month-old son, Angel, for 10 to 15 minutes. She heard about eight shots, then more. She lost track of how many.

Those "were the worst minutes of our lives," she said.

About 10 other people also sought safety in the waiting room.

Hundreds of police cars, firetrucks and ambulance encircled the hospital. Police blocked off streets in the surrounding neighborhood.

A message left for hospital officials was not immediately returned.

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.