At least 530 people in 38 states have been sickened by and seven died from a vaping-related lung illness, but it still is unclear what is causing the malady, officials at the Food and Drug Administration and Centers for Disease Control and Prevention said Thursday.

The two federal agencies are conducting parallel investigations to determine if the illness is related to legal vaping products as well as those available on the black market. To encourage information sharing on purchase of illicit products, FDA investigators from the Office of Criminal Investigations said they are not pursuing any prosecutions.

Those stricken by the illness all have a history of e-cigarette or vaping use. Some used just THC – the psychoactive ingredient in marijuana – or just nicotine or a combination of the two.

The situation has prompted calls for outlawing vaping products. Actions have been taken to eliminate legal vaping products containing flavorings attractive to minors like strawberry and bubblegum.

“The focus is on the supply chain,” said Mitch Zeller, director of FDA’s Center for Tobacco Products. “I would be very, very careful … about buying it [vaping products] in an alley or on the street.”

Marijuana industry experts say they suspect thickening agents used on the black market may be responsible. Both state labs and the FDA’s forensics lab have found the presence of vitamin E acetate -- a nutritional supplement meant to be used as a topical skin treatment -- in samples, but officials say they are not ready make any definitive statements. Some 150 samples collected from patients are being evaluated.

“Even though cases appear similar, it is not clear if they have a common cause or if they involve different diseases with similar presentations, which is an issue central to our investigation,” the FDA said.

“In many cases, patients reported a gradual start of symptoms, including breathing difficulty, shortness of breath, and/or chest pain before hospitalization. Some cases reported mild to moderate gastrointestinal illness including vomiting and diarrhea, or other symptoms such as fevers or fatigue.”

Half of the reported cases have been in people less than 25 years of age – two-thirds of them between the ages of 18 and 34 -- 75% of them male, the CDC said.

The CDC has said there’s no indication illness was caused by any infectious agent, making chemical exposure the most likely culprit.