More people could have been shot at Mercy Hospital last Monday if not for the quick action by police and the preparedness of hospital staff, who only recently had reviewed how to respond to an active shooter, experts say.

In the Nov. 19th incident, which unfolded over minutes, three people — Dr. Tamara O’Neal, pharmacy resident Dayna Less and Chicago Police Officer Samuel Jimenez — were killed by O’Neal’s ex-fiancé, Juan Lopez. Lopez died in a shootout with police.

Training for such situations has become routine, part of the regular schedule of annual workshops caregivers must undergo, thanks to accrediting agencies and medical associations.

Experts said the training is especially important at hospitals — and vital in emergency rooms. In those already-hectic places, doctors and other caregivers can find themselves trying to save their patients while also keeping themselves safe.

Monday afternoon, Mercy’s emergency department was shut down and evacuated, said Dr. Basem Khishfe, who works in emergency medicine at the hospital. A “code silver” — meaning an active shooter — was issued, and caregivers barricaded themselves and patients in offices and hospital rooms.

Dr. Michael Davenport, chief medical officer at Mercy, said during the code silver people were told to hide and take cover.

The hospital also has conducted active-shooter drills and training for employees, “but there’s nothing like when it actually happens,” Davenport said, adding there was nothing he’d have done differently.

“How does one prepare for catastrophe?” Davenport asked Wednesday. “You do the best you can, but when it happens, you react and there’s no wrong thing to do.”

Danny Chun, a spokesman for the Illinois Hospital Association, said training is required for active-shooter scenarios — as well as for biohazard, hurricanes and other emergencies — under regulations for hospitals receiving federal funds. Hospitals often stage mock incidents to figure out the best way to stay safe during a real shooting.

“Patients and family are the main concern,” Chun said. “If a shooting happens in the emergency department, caregivers are told to get to safety, evacuate the emergency department and lock it down.”

The procedures include not accepting additional patients, even from ambulances.

“Go to shelter. Issue a code. Stay in the office, turn off the lights,” Chun said. “That’s what you practice.”

Dan Regan, a spokesman for the Sinai Health System, said training for active-shooter situations has been part of the regular schedule for both Mt. Sinai and Holy Cross Hospitals. But preparing for those situations is an “ever-evolving process,” balancing care with safety.

“We’re constantly looking at ways to make sure we stay ahead of the curve, to make sure that our folks are prepared to deal with incidents they see on a daily basis. But they’re well-versed in these kinds of incidents because we’re a level-1 trauma center,” Regan said.

“Our priority is saving lives,” Regan said. “Our policies and procedures are geared toward making sure patients are protected and guests and caregivers are protected, too.”

The Joint Commission, which accredits about 77 percent of U.S. hospitals, requires hospitals to develop and implement testing and training programs and conduct drills, a commission spokeswoman said.

Chicago Police Department spokesman Anthony Guglielmi said the death toll could have been “much, much higher” if officers hadn’t responded so quickly.

Based on their behavioral profile of Lopez and “the amount of ammunition he had on him, we believe that he was intent on killing individuals inside the hospital. That was evident after he shot Dayna Less,” Guglielmi said.

“The first unit was on scene within seconds of him shooting Dr. O’Neal, and the tactical special operations response car was there within three minutes of the initial shots,” Guglielmi said in a statement. “Officer Jimenez came shortly after that.”

Gary Jackson, a behavioral psychologist and expert on preventing mass-victim attacks, has worked with the Secret Service and the CIA. He reviewed the incident and concluded that Mercy “did some very good things,” like training for active shooters, issuing an active-shooter code and quickly calling police.

Having an alert system like the “Code Silver” helped, but so did announcing specific instructions to help patients and visitors also know what was going on.

“Every time, it’s just tragic,” Jackson said. “This shooting was directed at [Lopez’s] ex-fiancee, but there was a police officer killed and a pharmacy resident, and that’s not unusual. He probably would have continued shooting anyone he saw.”