CHICAGO (CBS) — As a shooter gunned down three people at Mercy Hospital and Medical Center on Monday, hospital staffers and patients hid in offices, dove under tables and desks, and ran outside the hospital before the gunman either shot himself or was killed by police.

“People was hollering, screaming, scared, you know, nervous,” said Steve White, who was in the emergency room when he saw the gunman shoot a woman outside the hospital, before turning his gun on responding police, and bursting into the main lobby.

Dr. Tamara O’Neal, 38, pharmacy resident Dayna Less, 24, and Chicago Police Officer Samuel Jimenez, 28, were killed in the shooting. The gunman, Juan Lopez, 32, also died, although police have said it’s not clear if he shot himself, or if responding officers shot him.

Staff at Mercy Hospital completed active shooting drills less than two weeks ago, but employees said nothing truly prepares you for the real thing.

“We have prepared for an active shooter, just because of the world that we live in these days, but obviously never in our wildest imagination would we ever think that we would ever have to experience it, and today we have,” said Dr. Michael Davenport, chief medical officer at Mercy. “Our hearts go out to the lives that were lost, and we’ll all need healing, to say the least.”

The violence began in the parking lot, already too close for comfort, but then the gunman burst into the building. Witnesses said about 25 people were inside the emergency room when the shooter came in.

Surgery scheduler Patricia Rinella said her office is right next to the emergency room, so she told several people to come in before shutting the door.

“There’s about eight of us in there. There’s a lot of people running through,” she said. “Then we did hear ‘Code Silver, active shooter in the main lobby.’ So from there, he went into the main lobby, and it’s getting closer to us.”

White said the gunman didn’t look like he was going to stop.

“He was like he didn’t care about nobody, or nothing,” White said.

Police descended on the scene in full force, running towards the gunfire.

Witnesses inside the emergency room described screaming and chaos, despite pleas from employees and police to stay calm.

Hector Avitia and his wife were sitting in the hospital, waiting for test results, when they heard the gunshots.

“I turn around to my right, and I see a man shooting someone on the ground,” Avitia said.

Outside, some people hopped fences, others ran. More walked out with their hands in the air.

As the gunman made his way into the hospital, patients dove under desks to hide.

“One of the managers just told us to wait, because there were people outside, and they were shooting, and then all of a sudden they started screaming to get inside the offices, and to close the doors, and to keep silent, everything,” Maria Correa said.

Eventually, patients and staff were evacuated. Police set up a family assistance center at Dunbar High School to provide support for people separated from their loved ones during the shooting. Dunbar was briefly locked down after the shooting, but students were dismissed shortly before 4:30 p.m.

Staff at Mercy Hospital returned to work Tuesday morning, crime scene tape still lying in the snow outside serving as a reminder of Mondays tragedy.