(CNN) Chicago just ended its deadliest year in nearly two decades.

The city saw 4,331 shooting victims last year, according to its police department. Now, a new study sheds light on the medical treatment that such victims receive in Cook County, Illinois, the second most populous county in the nation, which includes the city of Chicago.

Almost one-fifth of gun-related injuries countywide that met the criteria for treatment at a trauma center, from 2009 to 2013, were initially under-treated, according to the study, which was published in the journal JAMA Surgery on Wednesday.

In other words, those patients received initial treatment outside a trauma center. Nearly one-third of all patients with gun-related injuries in the study were treated in a non-trauma facility.

"I was not surprised by the number of cases treated in hospitals without trauma units. ... What did surprise me was the number of firearm injuries," said Lee Friedman, associate professor of environmental and occupational health sciences at the University of Illinois at Chicago, and a co-author of the study.

The study involved an analysis of nearly 10,000 gunshot injuries in a single county over five years, he said, adding, "We have become numb to firearm-related injuries."

Other experts called the study important, but questioned whether it's sufficient enough to draw conclusions about the possible under-treatment of gunshot victims.

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Treating the wounded, without a trauma team

To assess whether a patient's injury was serious enough to warrant treatment in a trauma center, of which there are 19 in Cook County, the researchers took a close look at the patients' diagnosis codes in the data to learn more about the type of injury and the body region injured.

The criteria for a patient to be transported to a specialized trauma unit include the patient's level of consciousness, vital signs and location of injury, for instance whether a gunshot wound is to the head, neck, torso or above the elbows or knees, according to guidelines from the Centers for Disease Control and Prevention.

"I would say the vital signs actually supersede the location in terms of importance for most trauma surgeons," said Dr. Bryan Morse, assistant professor of surgery at Emory University School of Medicine and a trauma surgeon at Grady Memorial Hospital in Atlanta, who was not involved in the study.

The researchers found that some 28% of the patients were initially treated in non-trauma facilities.

Among those with injuries that met the criteria for treatment in a trauma center -- which were 4,934 -- the data showed that about 18% received initial treatment at a non-trauma facility, and about 10% of those were transferred to a trauma facility.

Morse said that he was surprised to see the number of patients that went to non-trauma centers, but added that the study had some major limitations.

'A good first pass'

"It's a good first pass at this data and to look at where firearm injuries are being treated, although that may be different from state to state, metropolitan area to metropolitan area, and county to county," Morse said about the study.

"One of the things they are missing is initial vital signs," he said. "So, not having that is a major limitation to the study and it really brings into question the validity of the study. Number two, they don't have any data on why people were transferred or why people weren't taken to a trauma center."

The study included data on 9,886 patients who were treated for firearm-related injuries at hospitals in Cook County from 2009 to 2013.

The data, which were derived from hospital billing records, included outpatients and inpatients, patient demographics, health outcomes and economic outcomes, but lacked physiological measures, such as blood pressure or respiratory rate, Friedman said.

Overall, limited resources and longer transport times to a trauma center may explain why some patients weren't treated in a trauma facility when they should have been, the researchers noted.

While the researchers did not examine racial disparities, the data showed that residents of disproportionately Latino or African-American communities in western and southern Chicago were more likely to be under-treated. These areas also have substantially fewer medical resources, Friedman said.

"This indicates a need for improved, better regional coordination between hospitals with and without specialized trauma teams," Friedman said.

Though the researchers noted that transporting patients to the nearest facility instead of a specific trauma center may explain their findings, the other factors that weren't clearly distinguished -- such as more details on type of injury -- could have influenced those findings, wrote Dr. Marie Crandall in an editorial commentary that accompanied the new study in JAMA Surgery.

For instance, a gunshot wound to the ankle would not necessarily need to be treated in a trauma center and so wouldn't count as the under-treatment of a patient. Whereas a wound to the knee would need a higher level of care, wrote Crandall, a professor of surgery at the University of Florida College of Medicine in Jacksonville, who was not involved in the study.

"It may be that 200 people walked into area hospitals after being shot in the hands or feet and were treated locally, which would be completely appropriate in the Chicago trauma care system," she wrote. "More work needs to be done before conclusions can be drawn or action plans can be discussed."

More likely to die now than 10 years ago

Some of the study's findings -- such as the importance of building a coordinated system of trauma care across communities -- could be applied to the rest of the country, said Dr. Angela Sauaia, a professor of public health, medicine and surgery at the University of Colorado Anschutz Medical Campus in Aurora, who was not involved in the study.

"The finding that does apply to the rest of the country is the fact that coordinated trauma systems must be designed using appropriate data if we are to improve trauma care," Sauaia said.

After all, victims of firearm injuries are more likely to die now than they were 10 years ago due to the increasing severity of gunshot wounds and the increasing number of patients shot multiple times, according to a 2016 paper written by Sauaia and her colleagues, which was published in the Journal of the American Medical Association.

"Another important finding is the astounding number of firearm injuries, the vast majority of which are completely preventable with appropriate prevention measures," Sauaia said. "Such as the involvement of primary care providers in providing appropriate screening and education on firearms."