BRISTOL, Wis. — The drug bust shattered the early-morning stillness of this manicured subdivision in southeastern Wisconsin. The police pulled up outside a white-shuttered brick condo, jolting neighbors out of their beds with the thud of heavy banging on a door.

What they found inside was not crystal meth or cocaine or fentanyl but slim boxes of vaping cartridges labeled with flavors like strawberry and peaches and cream . An additional 98,000 cartridges lay empty. Fifty-seven Mason jars nearby contained a substance that resembled dark honey: THC-laced liquid used for vaping, a practice that is now at the heart of a major public health scare sweeping the country.

Vaping devices, which have soared in popularity as a way to consume nicotine and THC, the psychoactive ingredient in marijuana, have been linked in the last several months to nearly 400 illnesses and six deaths. State and federal health investigators have not yet determined a cause, but authorities are focusing on whether noxious chemicals have found their way into vaping supplies, perhaps from a flourishing nationwide black market of vaping products fueled by online sales and lax regulation.

The bust this month in Wisconsin, where THC is illegal, offers an intimate look at the shadowy operations serving large numbers of teenagers and adults around the country who are using black-market vaping products, sometimes unknowingly because it is difficult to tell them apart from legitimate ones.