The first victim was conscious and described the carnage at the El Paso Walmart to doctors.

Within moments, though, the emergency room at University Medical Center of El Paso devolved into controlled chaos: “EMS called two, three, four, five, six” patients en route, emergency medicine doctor Nancy Weber recalled. “At that point, we knew that, yes, this was a mass casualty incident — yes, we were going to be getting a lot of patients.”

Suddenly, four operating rooms at the hospital were in use as teams of surgeons raced to save people at risk of bleeding to death from multiple gunshot wounds. Even for the only Level 1 trauma hospital for 280 miles, four surgeries at once is unusual.

“You just got to get in, stop the bleeding . . . then come back and fight another day,” said Alan Tyroch, chief of surgery and trauma medical director.

It kept going like that all afternoon, as patient after patient streamed in after a gunman went on a shooting rampage during the busy Saturday morning shopping hours at Walmart, killing 22 people and injuring more than two dozen others.By Sunday, they had treated 15 patients.

“I told everybody we’re going to be in this for the long haul,” Tyroch said. “This isn’t a sprint; it’s a marathon.”

The first notification went out about 11 a.m., shortly after the first 911 call alerting authorities to an active shooter situation. Many of the night surgeons were still doing morning rounds, and the day surgeon had already arrived. Surgeons began calling in specialists, residents, nurses — anyone they could find. Several came in without being asked after receiving the notification.

Tyroch was in Las Vegas for his mother-in-law’s 90th birthday when he received that first notification.

“Is this real? Are we getting patients?” Tyroch asked on a call with the hospital’s administrator on duty. Four or five patients were being brought in with more potentially on the way, the administrator told him. Tyroch began furiously texting faculty and his 18 residents, 15 of whom rushed to the emergency department.He jumped in a cab to the airport and made it back to El Paso within a few hours.

The medical center was ready for this, officials said. In October, it had participated in a citywide disaster training: a simulated mass shooter incident at the El Paso airport.

On Saturday, as patients poured in, there was no confusion, no panicking, no question over who was in charge of what. “We really were ready,” Tyroch said.

Of the 15 people brought in, 14 survived. One young woman was alive when an ambulance took her from the scene of the shooting but died within minutes of arriving in the emergency room, doctors said.

The hospital’s staff attended to the rest: victims with multiple gunshot wounds, victims with bullets in their chests, a man whose heart stopped and needed his chest opened to get it started again.

Doctors placed several tubes into patients to help them breathe, opened up two patients’ chests and treated complex orthopedic injuries from bullets.

The 14 victims required 109 units of blood. On a typical day, Tyroch said, doctors might use 10 to 12 units.

“Anybody who had skills to help came down, and we used every single one of them,” said Weber, the emergency medicine doctor who is also vice chair of quality and patient experience.

Del Sol Medical Center, another hospital within a couple of miles of the shooting, treated another 11 victims. One of Del Sol’s patients who required complex specialty surgery was transferred to University Medical Center, which is equipped to treat every aspect of an injury.

As of Monday afternoon, five patients remained in critical condition at University Medical Center with additional surgeries planned; five were in serious but stable condition; and two adults and two children had been discharged — including a 2-month-old infant boy whose parents had shielded him from a spray of bullets and were among the dead. He was treated for broken bones after his mother fell on him as she protected him.

Shortly before the shooting, a manifesto appeared online that authorities believe was written by the El Paso gunman in which he railed against a “Hispanic invasion.”

Just 13 hours after the El Paso shooting, another gunman in Dayton, Ohio, killed nine people and injured 27 others. President Trump addressed the country on Monday but said the focus should be on combating mental illness, rather than on implementing new gun-control measures.

For doctors at University Medical Center, there hasn’t been time to processany of that. Doctors performed five operations on Sunday. One patient had a major operation on Monday, Tyroch said. Another needs an eight-hour operation on Tuesday and will need additional hand surgery. Many patients will require extensive care and rehabilitation for days, weeks and perhaps monthsto come.

Despite the tragedy, the hospital, just miles from the U.S.-Mexico border, still needed to treat other patients. As victims poured in on Saturday, the hospital that night also attended to a migrant attempting to get over a border wall, who jumped off the wall and broke his leg.

Alexandra Hinojosa in El Paso contributed to this report.