On Sunday night, Toni Mullan drove 110 miles an hour on side streets from home to get back to University Medical Center of Southern Nevada, where she had just worked a 12-hour shift as a clinical supervisor in the trauma resuscitation department. Her car was smoking as she pulled into a three-hour parking spot close to the trauma center. Ms. Mullan, 54, left her hazard lights blinking as she shut the car door and raced inside.

Around a dozen patients had already arrived from the shooting scene at a music festival on the Las Vegas Strip. Two were dead and two had been “black tagged” — fully assessed and found to have injuries that were not survivable. Ms. Mullan’s daughter, Antoinette Cannon, 29, a trauma nurse who also works at the hospital, was standing out front with a physician assistant, taking injured patients out of vehicles as they drove up to the hospital and quickly assessing whose conditions were the most precarious.

“The minute I got there, I looked at the situation and said ‘How am I going to utilize my resources?’ ” Ms. Mullan, a registered nurse, said in a telephone interview Monday morning. By daybreak, 104 patients had arrived.

University Medical Center is the only level-one trauma center in Nevada and one of only a few free-standing trauma units in the nation. That means it is fully staffed with surgeons and trauma nurses day and night to handle injuries and mass casualties, from vehicle crashes that bring in 10 patients at a time to a 2015 episode in which a woman drove onto the Las Vegas Strip, sending 17 patients to the medical center. Last week, there had been 15 trauma cases in one night.

But even with 11 trauma bays, three operating rooms, a CT scanner, a trauma intensive care unit and a pediatric intensive care unit all under one roof, the trauma center had never faced a torrent like this. For two or three hours, the patients came nonstop. The radio at the clerk’s desk blared with transmissions from paramedics. With the frequency overburdened, other paramedics resorted to calling in patients by phone. Many patients simply arrived in cars or, in one instance, several in the back of a truck.

To an outsider, “it would look like a disaster zone, a chaotic scene, very chaotic,” Ms. Mullan said. “If a nonmedical person were to be sitting there watching this, they would think nothing was being accomplished.”

But in fact much was being done. It had been a busy day, which meant some of the day shift was still present and stayed to work alongside the night shift. Pagers went off with a be-beep be-beep each time a severe trauma case was identified. Patients, most of them with gunshot wounds, were doubled up two stretchers to a trauma bay. At one point, beepers screeched as five trauma cases were clocked in at the same time. “We couldn’t hear each other talk, it was that crazy,” Ms. Mullan said.

The injured also poured into other area hospitals. At Sunrise Hospital & Medical Center, about six miles from the concert site, 90 patients arrived in the first hours and by noon on Monday the total had reached 180. Of those, 124 “met the criteria for trauma activation,” said Dr. Jeffrey Murawsky, the hospital’s chief medical officer.

They included patients with single and multiple gunshot wounds to the head, face, chest, body, arms and, in one case, a finger. By Monday afternoon, sixteen patients had died, some on arrival and others after being treated.

Patients were sorted in the lobby of the emergency room on a scale of one to five, with one being the most critical, a system used daily. On Monday afternoon, operating teams at Sunrise were still at work. More than 50 patients had undergone surgery.

Hospital leaders should “think about the unthinkable, practice the unthinkable, even in a limited way,” Dr. Murawsky said.

Dignity Health-St. Rose Dominican received 61 patients at its three campuses, 32 of them to the Siena location, a level-three trauma center.

As of Monday evening, five of those patients were in critical condition, one was in surgery, and at least 22 had been discharged, said Jennifer Cooper, a spokeswoman. None had died. Most had endured gunshot wounds, but others had been trampled while fleeing, or had been hurt climbing fences to escape the gunfire.

Before patients came into the hospitals, medics had to extract them from the scene of the violence.

At 10:08 p.m., the call came to the Clark County Fire Department, which covers the area where the shooting occurred. Not knowing whether the gunman or gunmen had been neutralized, emergency workers wore ballistic helmets and body protection.

“They were exposed to significant trauma, things that are very very unusual — a lot of deaths, a lot of injury, a lot of hysteria in one place, a lot of tragedy, so quite frankly many of them will probably be dealing with this for the rest of their lives,” said Jeff Buchanan, a deputy fire chief with Clark County.

Ultimately, 108 fire personnel worked on the scene, including those from neighboring fire departments. Mr. Buchanan, who served at a command post, was later sent to the scene of the concert, where only the dead remained.

“It felt apocalyptic,” he said. “All the smatterings of blood, the blood footprints, just the pooling of bodily fluids, valuables strewn all over the place, phones, money, wallets, clothing just everywhere.”

Private ambulance companies staged in four intersections to the north, south, east and west. AMR Las Vegas and MedicWest sent 120 trucks and more than 250 personnel, said Damon Schilling, a community affairs manager for both companies. Some of the first to appear were the “walking wounded,” people who were able to escape and had less grievous injuries. Ambulance crews tried to focus on transporting critical patients to nearby hospitals.

At the trauma center at University Medical Center, workers rolled gurneys and wheelchairs outside so they wouldn’t take up space inside — a lesson Ms. Mullan said came from a recent training session with a doctor who had served after the Orlando Pulse nightclub shooting. Still, patients kept filling the trauma center. “We had them in the hallway, we had them everywhere,” Ms. Mullan said. “Wherever we could find a spot we did.”

The next step was to open up more places to receive them — a recovery room, a preoperative area, and an ambulatory surgery center were staffed and made ready. A separate place was set aside for the patients with unsurvivable injuries to receive comfort care until they died. Two areas were established for patients with injuries that were not life-threatening. With staff and volunteers pouring in, these patients, too, received immediate care from a dedicated doctor.

Teams reassessed intensive care patients, moving the less critical to make room for others.

The trauma center repeatedly ran out of supplies including IV tubing and fluids, chest tubes, blood pressure cuffs, urinals and blankets, and “begged, borrowed and stole” them from other departments of the hospital.

The cafeteria was turned into a respite area for families searching for loved ones. Many patients arrived at the hospital and others as unidentified “Does.” Ms. Mullan said as of Monday morning that the list of which patients were where was “still a work in progress.”

In the end, Ms. Mullan said, the systems in place were able to flex with the extra burden and bring care to those who needed it. In the midst of the disaster, the trauma center even took in a burn victim and a pedestrian hit by a motor vehicle.

For other hospitals that may be called on in future mass shootings, Ms. Mullan had advice. “Be prepared and practice,” she said.

As she remained at the hospital late Monday morning, Ms. Mullan reflected on all the people who arrived to help, including workers who were not on shift and every hospital administrator.

“I’ve been a nurse for 30 years, and this was by far the worst moment I’ve had, the worst injuries,” she said. “But it was the proudest moment.”