Transcript for Inside one of Texas' largest mass casualty drills
units -- Reporter: For these first responders in San Antonio, Texas, every second counts. Hurt, where's hurt -- Reporter: Every move on the scene, every decision at these trauma centers, is the difference between life and death. We're going to need someone to continue holding pressure on the neck. Reporter: The trauma may look disturbingly real, but this is a drill. What is happening today is one of the largest mass casualty drills in the nation. It's a nightmare scenario. Three active shooters opening fire and detonating a bomb during a high school graduation at the Freeman coliseum. The details may be different but the aftermath is created to look eerily similar to the recent mass shootings. Las Vegas. Pittsburgh. Thousand Oaks. And just one year ago, here in Texas, the church shooting in Sutherland springs. It's not a question if something's going to happen, it's a question of when. How we're prepared to respond to that is critical. Reporter: The drill starts before sunrise. I am working on a deep laceration to the arm -- Reporter: Professional makeup artists with volunteers who are playing victims. It's important to make it realistic. When actual medical personnel run into something that is gruesome, they'll be better prepared to deal with it. Reporter: Each of these actors has been assigned a specific trauma. Do you know when you're supposed to do as a person with an arterial gunshot wound? It says I'm supposed to scream in pain. Reporter: First responders begin to take their places. They should guide you in. Reporter: Lieutenant John Franzen preps his team. We're going to run it like a real event. Reporter: As the mock shootings begin to unfold -- They're dispatching right now. Ladder 9, engine 9, engine 30 -- Reporter: Franzen gives us an inside look at the very first moments of the fire department's response. What we don't want to do is send all of our assets to one location, then it be a complex coordinated attack. Reporter: Again, this is just a drill. But every detail matters. This is the best practice they will get for real-life trauma. You can get a sense of the chaos of a moment like this. Helicopters up in the air. Ambulances on the ground. Medical supplies everywhere. Reporter: The mass hysteria is a planned response. S.W.A.T., fire and rescue, paramedics who are S.W.A.T. Trained as well. They're trying as quickly as possible to get people who need immediate treatment out first, and those who can survive a bit longer, leave them for the end because they don't need the help as immediately as others. Figuring out who goes where and how they're going to get there is one of the biggest hurdles for first responders. Dsw, she's got to go next. Reporter: Gsw is gunshot wound. This young lady has one, as does this one. These patients need to go next, they need immediate treatment. Right now we're going to join the casualties in one of these ambulances going in. And we're going to guide out to the heli pad. Reporter: The most severe victims are airlifted to local hospitals. This wounded officer being driven by ambulance to a nearby helipad. He's got a significant wound to the head with a gunshot, one to the hands. You can tell how rocky it is in an ambulance like this, moving around. These paramedics have to work under these conditions. Stuff falling down everywhere. It's not easy. Meanwhile, patients from the coliseum begin flooding the ers of some of San Antonio's level 1 trauma centers. Brook army medical center and university hospital. Going into the bay. Go ahead and get her oxygen. It's chaotic but controlled chaos. Reporter: Dr. Ramon sisterer in is the trauma surgeon on call today. He says the purpose isn't just to run through the medical treatment. What do we do after this? We get x-rays. Reporter: Also to streamline more basic things like communication between different wings of the hospital so a patient can get critical care faster. We called O.R.? We called O.R. What did they say? You physically have to call the O.R. Communication is a big piece of what we're going to do. Trying to figure out how they'd manage it at our level, at their level. Optimizing that is one of the key goals for today. To be road to receive a mass casualty incident, at 2:30 on a Saturday morning, 3:00 A.M. On a Wednesday -- Reporter: As doctors a know all too well, it could be on a quiet Sunday at 11:20 A.M. That's when a gunman armed with an ar-15 walked into the first Baptist church in Sutherland springs, Texas, and opened fire. Killing 26 and injuring dozens. Dr. Ronald Stewart was one of the trauma surgeons working at university hospital that day. We treated nine patients. Did all of them survive? One did not. One did not. Small child. I think the average person would probably not understand, really, how difficult it is to take care of patients with these types of high-velocity wounds. What you are about to see might be disturbing. Basically this is really very similar types of wounds that we saw at the Sutherland springs. Reporter: Dr. Eastridge uses snapshots from a real war zone as an example. This here is actually what will be a typical wound from a high-velocity round. It basically creates a wave, a blast wave. Like if you throw a pebble in a pond. Reporter: This is why, in addition to practicing how they respond to a mass shooting, both Dr. Eastridge and Dr. Stewart believe their medical community must also get involved in curbing firearm violence. We really have put our heart and soul into reducing these injuries from occurring in the first place. Reporter: But unsurprisingly, this view is controversial. When the American college of physicians recently published this paper on the topic of gun violence, the NRA responded by telling doctors to say in their lane. Then medical professionals across the country fought back. Using the viral hash tag #thisisourlane, sharing gruesome images of their firsthand experiences with gun violence. Dr. Eastridge says his own experience with gun violence, from the Sutherland spring shooting, took an emotional toll on his hospital. Everybody really collapsed later that afternoon. Do you remember that moment for you? I made it to my truck. And then I probably didn't leave for about an hour. I mean, that illustrates how we compartmentalize it. We all shed a lot of tears later that day. And days subsequent. Reporter: For everyone involved in today's drill, this is personal. Because whether they've already faced tragedy before or fear that they may have to, they know that what they're learning today will save lives. You have all of these resources available, it's going to help us when the real deal happens. Reporter: For "Nightline," I'm Matt Gutman in San Antonio, Texas.
This transcript has been automatically generated and may not be 100% accurate.