LOS ANGELES — Doorways chained shut and “Members Only” signs warn casual passersby against stopping along this five-block stretch of downtown known as the “Cannabis District.” This gritty corridor is a major hub for the estimated $9 billion black market for the state’s illicit cannabis products.

Products sold here, including a flood of counterfeit vape materials from China, are coming under scrutiny as federal authorities investigate the mysterious vaping-related lung illness that has sickened at least 530 people in 38 states and claimed nine lives.

Many sick patients said they bought vape products containing THC, the psychoactive component of marijuana, on the black market, officials and clinicians said. The biggest legal marijuana market in the world is California — and the black market there is three times as big, said David Abernathy, an executive with Arcview Market Research, a cannabis investment and market research firm.

“There’s a huge market for what are now illicit cannabis goods, and they tend to be much cheaper than their legal counterparts,” Abernathy said.

Interviews with more than a dozen business people in both the legal and illicit cannabis markets, as well as officials and clinicians, point to recent changes in the ingredients used in popular marijuana vaping devices that could be making people sick. They say black market operators are using more thickening agents to dilute THC oil because of a crackdown by state authorities that has made the oil scarcer on the black market. THC oil is used to fill tiny disposable containers known as vape cartridges, which are heated to create inhalable vapor. Vaping cartridges are among the most popular items in the legal and illicit markets, industry analysts said.

Industry experts said that many of the unfilled cartridges come from Chinese factories that follow the Instagram accounts of well-known brands and churn out copies by the thousands. Black market manufacturers buy the cartridges in markets like this, fill them with THC oil or other mixtures and package them for sale by distributors.

As part of the national investigation into the spate of illnesses linked to e-cigarettes, the U.S. Food and Drug Administration disclosed that it had launched a criminal investigation with the Drug Enforcement Administration. That is happening alongside the probe by the Centers for Disease Control and Prevention into the cause of the illnesses that began being tracked in April. FDA acting commissioner Norman “Ned” Sharpless told a House panel Wednesday that officials are not pursuing individual vapers. But if the FDA determines “someone is manufacturing or distributing illicit, adulterated vaping products that caused illness and death for personal profit, we would consider that to be a criminal act,” he said.

Recreational marijuana use is legal and regulated in California. In recent months, tighter restrictions have made THC oil increasingly scarce on the black market, industry experts said. THC oil can cost between $5,000 and $7,000 per kilogram for a high-purity product, according to experts. It is thick, colorless and odorless.

To stretch that supply, “a lot of producers who are using thickeners in their mixtures, they’re buying this stuff because it creates the visual appeal of a very thick oil” that looks like THC oil, said Peter Hackett, owner and operator of Air Vapor Systems, a company based in the San Francisco area that provides vape hardware and supplies to the legal market.

One thickening agent that is a key focus of investigators is vitamin E oil, known as vitamin E acetate. It is also colorless and odorless, has similar viscosity to THC oil, and is much cheaper.

The sign of THC purity on the black market is oil thickness, Hackett said.

“It’s a way to fool or trick a customer into buying a bad product,” he said.

Vitamin E acetate is commonly used as a nutritional supplement and in skin-care products. It is sold legally. It’s not harmful when ingested or applied to the skin. But health officials have warned that, based on its chemical structure, it could be hazardous when inhaled, potentially causing the sorts of symptoms many patients have reported: cough, shortness of breath and chest pain.

One popular brand of additive, known as Honey Cut, is available for sale by the barrel here, said shop owners and business operators in the legal cannabis market. The Santa Monica, Calif.-based company that sells Honey Cut is one of three firms recently subpoenaed by New York state health authorities seeking more information about their ingredients. New York state’s Wadsworth Center lab found products from all three firms to be “nearly pure vitamin E acetate oil,” according to New York Gov. Andrew M. Cuomo (D).

Honey Cut Labs’ chief executive, Joshua Temple, did not respond to several requests for comment via email and telephone.

Shopkeepers here say that counterfeit products from China are also being sold as Honey Cut, adding to the confusion about what vapers might be inhaling.

In California’s legal marijuana market, products are tested for potency of THC and the presence of heavy metals, residual pesticides and other substances, but not for vitamin E acetate.

No one product or substance, including vitamin E acetate, has been conclusively identified as the cause of the lung injuries. But vitamin E acetate has been found in THC products taken from sickened patients and tested by state labs and the FDA’s forensic lab, officials said.

In Utah, 90 percent of THC products tested in the past several weeks contained vitamin E oil, according to the state health department.

“We haven’t found anything out of the ordinary with any of the nicotine samples,” department spokesman Tom Hudachko said.

All the reported cases in the national lung disorder outbreak involved a history of e-cigarette or vape use, CDC officials have said. Most people used e-cigarette products containing THC, many of them illicit products, officials said. Many people said they used both nicotine and THC, while some people reported using only nicotine products, the officials said. Jennifer Layden, Illinois’s state epidemiologist, noted, however, that there is often “hesitancy about sharing information” if patients used illicit products.

Looking for vitamin E acetate

Some companies selling cannabis products are already marketing them as free of vitamin E acetate. Cannabis testing labs are also developing methods for testing vitamin E acetate in tobacco and cannabis products. One company, Cannalysis Labs, said it plans to make its methods publicly available so other labs can use them to “reduce the hazards of these products that are being consumed,” said spokeswoman Chi Zhao.

Little is known about the potential hazards of vitamin E acetate. But talks about potential animal testing have begun at the National Institutes of Health, public health experts said. “Nothing has been defined or formally put in place at this point,” said NIH spokeswoman Renate Myles.

“We really don’t have the information, and having animal models may lend some insight,” said Ruth Lynfield, Minnesota’s state epidemiologist. “We are really interested in vitamin E acetate.”

Scott Aberegg, a pulmonologist and critical-care specialist at the University of Utah hospital, has seen more than a dozen patients with severe vaping-related lung illnesses in recent weeks. He has begun testing them for excess levels of vitamin E in the blood. Finding very high levels may suggest that it or other contaminants that may be mixed with it are causing patients to get sick, he said.

On the flip side, because it is not clear how vitamin E is absorbed by the body, low or normal levels of vitamin E may not necessarily mean anything, said fellow pulmonologist Sean Callahan.

“This could all be a shot in the dark, because we’ve never seen anything like this before,” Callahan said.

California’s cannabis market supplies much of the nation. Abernathy, of the cannabis investment firm, said that 80 to 85 percent of all marijuana grown in the state is shipped elsewhere. “It’s absolutely enormous,” he said of the market.

High taxes on legal marijuana and new state regulations put in place last year have fueled an even stronger black market, he said. Many businesses that were unable to comply with stricter regulations chose not to become licensed.

“A lot of businesses had cannabis that wasn’t compliant under the new regulations,” said Abernathy. Some of that winds up on the black market, he said.

One illicit operation recently uncovered by Wisconsin law enforcement authorities involved what they described as a massive drug ring that produced 4,000 to 5,000 counterfeit vaping cartridges filled with THC oil every day for almost two years.

Authorities allege that Tyler T. Huffhines, 20, who they charged as the ringleader, flew to California in late August to buy “forty to fifty jars of THC distillate to manufacture additional THC cartridges and he was going to have the jars sent to Wisconsin,” according to a criminal complaint filed this month in Kenosha County.

Over Labor Day weekend, Huffhines allegedly flew to California again, this time with over $300,000 in cash to buy additional distillate, the complaint said.

At the Kenosha County condo where his operation was based, authorities seized more than 31,000 pre-filled one-gram THC vape cartridges, about 98,000 unfilled vape cartridges, 57 Mason jars of “refined liquid THC,” and thousands of empty vape cartridge boxes and packaging, among other paraphernalia, the complaint said. The estimated street value of the THC products exceeds $1.5 million, authorities said.

Kenosha County Sheriff David Beth said the THC products are being tested to see if they can be tied to the vaping illnesses. In Kenosha County Circuit Court last week, Huffhines’s attorney, Mark Richards, said there is no evidence connecting his client to the lung injuries, the Kenosha News reported.

“There have been no deaths in Wisconsin,” Richards said, calling statements the vape cartridges could be tied to illnesses and deaths “hysteria.”

Products filled with 'trash'

Here in downtown Los Angeles’s Cannabis District, the shops wedged between the homeless tent city of Skid Row and luxury condos in Little Tokyo have names such as “Doctor Vape,” “Smoke Tokes” and “Cart Cartel.”

Inside many of the stores are shelves stacked with ready-to-fill cartridges and empty counterfeit packaging that copies the most popular vape cartridge brands, said hardware supplier Hackett. Virtually all of it is manufactured in Shenzhen, China, he said.

In the past year, Chinese companies have also opened offices in Southern California, including the Los Angeles area, because of the lucrative vaping market here, he added.

Sidewalks outside the storefronts double as ad hoc loading docks. One shop representative who agreed to speak on the condition of anonymity because he feared retaliation from other store owners, said more business people from China have come to the district in the past four months to sell counterfeit Honey Cut thickener in large quantities.

“We know for an absolute, 100 percent fact that the Chinese were coming here and selling products called Honey Cut on our blocks by the barrel — barrel after barrel after barrel,” the shop owner said. “We didn’t see any problems until we started seeing the Chinese knock this off.”

On a recent Saturday, two men leaving a shop with a box of products confirmed that thickening agents, including Honey Cut, are widely available for sale.

“Honey Cut, yeah. That’s the biggest cutting agent. Honey Cut,” said one of them, a heavily tattooed man who asked that only his first name, Roger, be used.

The men said they do not use cutting agents or diluents in their products, which they sell outside the area.

“A lot of people are just filling these pens or vapes or whatever with trash,” Roger added.

The other man, Timmy, who made the same request as Roger, agreed. “It’s hurting somebody just for money,” he said.

Sun reported from Washington. Alice Crites contributed to this report.