CHICAGO (CBS) — The morning after four people were slain at Mercy Hospital and Medical Center, employees faced the somber task of returning to work where two colleagues lost their lives.

Dr. Tamara O’Neal, an emergency room physician, and Dayna Less, a pharmaceutical resident, were among the four people gunned down at Mercy on Monday afternoon.

tamara oneal and dayna less Its Going To Be A Very Somber Day For Us; Mercy Hospital Staff Mourns Slain Colleagues

Dr. Tamara O’Neal (left) and pharmaceutical resident Dayna Less (right) were shot and killed at Mercy Hospital on Nov. 19, 2018. A Chicago police officer and the gunman also were killed in the shooting. (Photos supplied to CBS)

Police have said 32-year-old Juan Lopez gunned down O’Neal, his former fiancée, in the parking lot outside Mercy Hospital. He opened fire on police officers as they arrived at the hospital, and then went into the lobby, where he Less as she was exiting an elevator. He also shot and killed CPD Officer Samuel Jimenez, one of the officers who confronted him inside the hospital.

sam jiminez Its Going To Be A Very Somber Day For Us; Mercy Hospital Staff Mourns Slain Colleagues

Samuel Jimenez was killed Monday by a gunman at Mercy Hospital.

Tuesday morning, the emergency room at Mercy Hospital was still closed, but hospital employees headed back to work to care for patients at the hospital.

Ribbons, teddy bears, and crosses were set up at the edge of the hospital campus in honor of O’Neal and Less.

A charge nurse who worked alongside O’Neal described the difficult shift ahead.

“It’s going to be a very somber day for us. It’s on our minds, and we just don’t know how to deal with this, but we’re a very tight department. So we’re all going to be there for each other, and support each other,” Cindy Centeno said.

Lopez also was killed in the shooting at the hospital, but police said they do not know who fired the fatal shot, Lopez or one of the officers at the scene.

juan lopez Its Going To Be A Very Somber Day For Us; Mercy Hospital Staff Mourns Slain Colleagues

Juan Lopez, 32, shot and killed three people at Mercy Hospital on Nov. 19, 2018. Chicago police said Lopez also was killed. (Source: Facebook)

A Chicago Housing Authority worker, Lopez had been employed since February as an associate program specialist for the customer care center at CHA headquarters.

CHA officials said he passed a background check, and had no history of complaints while he worked for the agency.

“This was an unthinkable act of violence. We are filled with overwhelming sadness and our hearts go out to the families of those who died, and to all affected by this horrific tragedy,” CHA chief executive officer Eugene Jones Jr. stated in an email.

The Cook County Medical Examiner’s office was expected to perform autopsies for Lopez and the three victims on Tuesday.