With the death toll rising, the FDA has enlisted criminal investigators to determine how the health hazards of vaping may be leading to illness.

Earlier this summer, doctors started to recognize similar symptoms in many of their patients who had been admitted for respiratory distress. Many patients were admitted to the hospital and put on respirators for coughing, chest pain, fever, fatigue, decreased lung capacity, and gastrointestinal distress. Imaging of many of the affected patients’ lungs showed damage, but no signs of infection.

Eventually, doctors started to realize that these patients had one common factor: vaping.

“Once you see the connection, you can’t unsee it,” Frank Leone, a Penn Medicine pulmonologist, told the Wall Street Journal. “The assumptions that would have guided care a month ago no longer do.”

A shocking 530 cases of the mysterious vaping illness have been reported, according to the U.S. Food and Drug Administration (FDA). Three quarters of these cases reportedly involved male patients, and two thirds of the patients were between the ages of 18 and 34.

The health hazards of vaping have become more debated in recent months, with a close focus on teen vaping usage. Although increased media attention has been on the teen vaping epidemic, only 16 percent of the victims affected by the mystery illness are younger than 18.

As of Sept. 23, nine people have died from the illness with the latest death occurring in Kansas. The most recent death, reported by NBC, occurred in a man over the age of 50 who had underlying health issues.

Other deaths have occurred in California, Illinois, Indiana, Minnesota, Missouri and Oregon.

In light of the growing problem, the FDA will reportedly be utilizing its criminal investigators to probe the issue. The agency notes that they will not be pursuing individuals or companies based on the health hazards of vaping, but are simply trying to figure out the cause before more people die.

The FDA has collected over 150 samples of vaping fluid from numerous states. These samples will reportedly be tested for nicotine, THC, toxins, pesticides, and other chemicals which may be added for cutting or additive reasons.

Previously, analysis found that some samples contained vitamin E acetate. However, this has not been consistent across all samples. In fact, there has not been one ingredient that has shown up in every sample tested by the agency.

Unfortunately, the unknowns make the investigation a complex matter. It has been hard for investigators to pinpoint a common thread other than vaping, leaving regulators and doctors at a loss for what to do next.

“I’d like to stress how challenging this situation is, as patients may have been exposed to a variety of products and substances, may not know the contents or sources of these products, and in some instances may be reluctant or too ill to disclose all the details of interest,” Anne Schuchat, principal deputy director of the U.S. Centers for Disease Control and Prevention, told USA Today.