State and federal investigations into the puzzling burst of severe lung illnesses linked to e-cigarette use—aka vaping—are focusing in on black-market and counterfeit products, according to a report by the Washington Post.

Unknown adulterants and dubious solvents—such as oils and diluting “cutting agents”—in vaping liquids are now the prime suspects behind the illnesses, which have has led to 215 possible cases in 25 states . One person in Illinois has died. Investigators say that in many of the cases, people bought suspect products on the black market or in “pop-up” shops.

Solvents in counterfeit and black-market vaping liquids “can vary a lot,” an unnamed official at the Centers for Disease Control and Prevention told the Post. The official added that solvents sold for mixing home-made vaping liquids may also be mislabeled.

“What’s likely causing the harm is something that they are putting in to make it easy or cheap to mix,” former Food and Drug Administration commissioner Scott Gottlieb told the Post.

Gottlieb, who led a crackdown on e-cigarette makers while at the FDA, said that mainstream e-cigarette products such as those from Juul or Blu are unlikely to be involved in the cases. While such products may cause chronic problems, he said, the recent flare-up of cases involve acute illnesses—ones that haven’t been seen before and are spread unevenly across the country.

Abrupt illnesses

Those sickened often suffer gradual breathing difficulties, coughing, fatigue, chest pain, and weight loss, which leads to hospitalization. Some have also experienced vomiting and diarrhea.

Many cases seem linked to vaping liquids containing THC, or tetrahydrocannabinol, the primary psychoactive ingredient in marijuana. Investigators say that they’re also looking into suspect nicotine-containing liquids.

Shady THC-containing liquids seem to be the primary suspects in investigations in several states, including Utah, Pennsylvania, and California.

“We suspect adulterated or contaminated products, because these [marijuana] products have been out there for some time, and we’ve not seen these cases until this summer,” Phillip Lamberty, a pulmonologist and critical care specialist at the University of Pittsburgh Medical Center (UPMC), told the Post. Lamberty treated three vaping-linked cases recently, at least two of which were linked to THC-containing products. One was bought online and the other from an illicit drug dealer.

Sketchy sales

The UPMC health system, which includes 40 hospitals, has seen at least 14 patients with vaping-linked illnesses. Several patients said they bought the black-market brand "Dank Vapes" products online.

In California’s King County, health officials linked all seven cases in the county to “pop-up” shops selling marijuana vaping cartridges.

“The patients had switched from regular retailers to the pop-up shops,” elaborated Nancy Gerking, the county’s assistant director of public health. The patients “found a difference between the potency of the products,” she added. “They had to use twice as much, so they were taking twice as much of the product into their lungs.”

Some health officials have suggested that consumers stop using all vaping products until the culprit(s) are clearly identified. However, e-cigarette makers and other health experts have pushed back, noting that the issue is clearly with dubious products, not vaping generally.

This post has been updated to include the latest case numbers.