Roughly half of the 58 victims killed in the Las Vegas mass shooting died before reaching a hospital, based on mortality counts released by local medical centers. In the Iraq and Afghanistan wars, there were also many pre-hospital fatalities. “It’s convenient to put many of these deaths in the damn-shame category — ‘Well, we couldn’t have done anything anyway,’ ” Dr. Rasmussen said. While that reaction is understandable, he said, it was important to study what really caused the deaths, and whether any of the injuries might have been survivable.

In studying autopsies of service members killed in the early 2000s, Dr. Rasmussen and his colleagues found that many had life-ending wounds that could not be fixed. But some had wounds that could have been successfully repaired by surgeons if the patient had not already lost so much blood, he said.

Because of that finding, the military began training regular service members to stop bleeding, equipping them with individual first aid kits, which contain optimized tourniquets and wound dressings. Researchers have shown that lives have been saved as a consequence.

“We think the American public can, if they can use some of these same techniques, save lives in the same way nonmedical soldiers can save lives,” said Dr. Craig Goolsby, deputy director of the National Center for Disaster Medicine & Public Health at the Uniformed Services University.

The impression in Las Vegas is that they did just that. “Some of our patients were saved by bystanders,” said Dr. Deborah A. Kuhls, a professor of surgery at the University of Nevada, Las Vegas School of Medicine and the medical director of the trauma intensive care unit at University Medical Center of Southern Nevada. “One had put a belt on his thigh, and it probably saved his life. There was a lot of bleeding on the scene.”

In Las Vegas, several factors impeded the arrival of emergency medical workers at the scene of the shooting itself.