LOS ANGELES — Using brain scans and exercise stress tests, researchers have identified two biologically distinct subgroups of veterans suffering from “Gulf War illness.”

Their bodies reacted differently to physical exertion, and their brains had atrophied in different regions. None of the patterns were seen in a control group of healthy subjects.

The findings, published online last week in the journal PLOS One, are part of a growing body of work that the authors said could eventually lead to biological markers for the mysterious condition, which is still defined by its hodgepodge of symptoms.

“That’s the hope,” said Georgetown University researcher Rakib Rayhan, lead author of the study.

Still, the importance of the differences his team identified is far from clear, said Dr. Beatrice Golomb, an expert on Gulf War illness at UC San Diego, who was not involved in the research. There are many ways to parse any population of patients with a condition that is so variable and diverse, she said.

After the 1991 Gulf War, veterans began complaining of various problems, including pain, fatigue, headaches and cognitive impairment. The symptoms ranged from mild to debilitating.

Up to 30 percent of the 700,000 troops who served in the war are thought to be affected.

While exposure to nerve agents, pesticides and smoke from burning oil wells have all been deemed possible culprits, no definitive cause has been identified.

To the surprise of the researchers, two groups of veterans emerged in the analysis. In 18 of the veterans, pain levels were elevated after exercise. Their scans showed a loss of brain matter in the regions associated with pain regulation.

In the other 10 veterans, exercise triggered a condition in which moving from lying down to a standing position causes the heart to race. They had atrophy in the brainstem, which controls heart rate and blood pressure.

The two groups also had distinct patterns of brain activity during cognitive testing.