Jorge Sainz was born in central Mexico, nearly five decades ago. After obtaining a green card, he moved to Texas, in 1991, as a U.S. resident. He studied medicine at the University of Texas at San Antonio, started his residency in Buffalo, and, in 2003, obtained his U.S. citizenship. “Becoming a citizen is more than a piece of paper,” Sainz told me, on Sunday morning, in the El Paso offices of Pininos Pediatrics, where he is the chief doctor specializing in pediatric critical care. “I needed to understand exactly what I was getting myself into before I said, ‘O.K., I want to be a citizen.” As we talked in his office, half a mile from the U.S. border with Mexico, he offered me coffee and cookies before describing his role in tending to victims of the mass shooting nearby that killed twenty people on Saturday. “Honestly, I’m still trying to understand what I got myself into,” he said, referring to his decision to move to the U.S. “But I came here for hope, and I became a doctor to give that back. The more you give, the more you get.”

Now forty-seven years old, Sainz, who wears glasses and combs his graying hair straight back, has been practicing medicine for nearly fifteen years in El Paso, which he described as “the best of both worlds—living here and living there,” as he gestured back toward Mexico. He routinely cares for people from both sides of the border, including migrants fleeing from violence, who are suffering, he said, “from everything in the medical books.” Many are unaccompanied minors. Most are, he said, “very respectful and very determined.” Sainz treats them all the same. “They remind me of me,” he said, “because my family came through this country looking for a new beginning, too.”

“It’s our job to care for everyone,” Sainz added, shrugging, “whether we get paid or not.”

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In that respect alone—caring for patients, regardless of their origin—Saturday afternoon was like many before it for Sainz. Shortly before noon, he was driving to Del Sol Medical Center, a few exits down the highway from Cielo Vista Mall, to “visit with newborns” on behalf of a pediatrician friend who was out of town. A motorcycle sped past him, followed by state troopers. A helicopter buzzed overhead. He figured it was a high-speed chase. Inside the hospital, as he began tending to babies on the fourth floor, nurses started talking about a mass-casualty shooting nearby. He later learned that a white, twenty-one-year-old man named Patrick Crusius, from a Dallas suburb, had killed at least twenty people in and around a Walmart near the Cielo Vista Mall, injuring twenty-six others. Sainz described it to me as “a rich target. A very crowded area with many, many people from across the border, especially at the Walmart. Especially on a Saturday morning. Many times, you won’t hear a word of English there.” Sainz’s account of what happened at Del Sol—which treated eleven victims of Saturday’s mass shooting—has been edited and condensed.

“I heard an announcement: ‘Any doctor, could you please come down to the E.R.’ So I said, O.K. I didn’t know if there were maybe some kids that were going to be brought over there. But I thought, Maybe I can be of some help.

“Everybody is yelling, everybody is trying to get orders. I see the main trauma surgeon rushing around. He had already triaged some of the patients, and they were already moving them to the operating room so that they can make room for the next ones, the less sick ones, waiting in the hallway. Usually there are one or two people at these kinds of centers who are ready to deal with escalating trauma, but this was unprecedented. There is civil trauma and then there is military trauma, the stuff that happens at war. Ninety per cent of them die at the scene; maybe ten per cent survive. This was getting close to military trauma. This guy wasn’t shooting a .22 or a little rifle. I was seeing scooped-out flesh. It kept coming. And coming.

“I’m a pediatrician, right? Well, for a while there, me and a maxillofacial surgeon—a very good dentist, basically—were running the show, coördinating care in the trauma ward. The few surgeons they had were already in the operating room. There is this very rigorous, quality-driven process where you get a privilege to go to the hospital. Well, people were operating yesterday with no privileges, because it was an emergency situation.

“There were three people that I was taking care of at the same time. Somebody showed me a video later, of the shooting, and I could see some of them down on the Walmart floor.

“We give fictitious trauma names to patients so they don’t get mixed up if they have the same or similar names. A machine spits them out. Some systems offer flower names for women and car names, like Cadillac or Mustang, for men.

“All three spoke Spanish. All three were Americans. All three were critical. The woman was thirty-five and bleeding a lot. One man was also thirty-five and had been shot in the leg and had a pelvic injury. Another man was fifty-five and had been shot through the abdomen, and the bullet came out his buttocks. They were all able to communicate somewhat, but they also were all bleeding profusely. I was administering a lot of blood products, doing mass transfusions with these boxes of fresh blood. If you wait, they die. You guesstimate how much blood is lost, guesstimate vital signs, and start giving it. The thirty-five-year-old man would have probably died most quickly without the blood.

“I spoke to them in Spanish, which they preferred. I just introduced myself. I said, ‘My name is Dr. Sainz. You have some injuries. We’re treating you. You’re going to be O.K. We’re here to help you. Please let us know if you have any pain, or anything else that you want us to know.’ We asked if they had any relatives that we could communicate to that they’re alive and there. Every so often—every five, seven, ten minutes—I’d go up to basically whisper to them, during the chaos, and tell them, ‘We’re still working on things. Things are moving in the right direction.’ Hopefully.

“They’d just nod and say thank you. That’s what I remember.

“Then we’re pushing them through CT scanners. Everyone’s yelling. Everybody’s administering blood, carefully. Everybody is signing these pieces of paper so there will be a track record at the end, so we can try to make some sense of where the blood went, who got it, all this stuff. We’re writing on napkins.

“At some point, a rumor comes in, amid the chaos. The other hospital where patients went—the mall is halfway between two, and some patients went left and some went right—well, we heard there was an active shooter there. Hospitals are soft targets. I start thinking, This might be more organized than the usual shootings. I asked someone to call the other hospital and ask if it was real and he was, like, ‘No, no, no.’ I said, ‘Well, we need to move law enforcement here, right now, anyway, and lock this place down.’ And they did.

“Resources arrived gradually and were all in place about four hours later. Backups arrived. I walked away. I went back to babies, in another hospital. I was in bed by four in the morning. Up by six-thirty. Normal.

“If you go back one hundred and eighty years—when Texas was actually Tejas—people were crossing the river to come to Tejas, fighting for government representation, to speak English, to not be Catholic. They were fighting for equal rights back then, saying, ‘We are Tejanos, too.’ And here we are now. Saying the same thing! And somebody feels empowered to take someone else’s life, because they don’t look like Texans to him. Well, we are all equal. We’re all equal. And we’re all vulnerable.”