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The address was posted on a Friday night and visible only to a select group of people: the followers of a private Instagram account.

It was a darkened street in downtown Los Angeles. Not long after an NBC News producer arrived, he noticed a figure rushing toward him, a man wearing a bulletproof vest with a handgun strapped to his hip.

"Follow me," the man said.

He walked into an alley and through a parking lot, stopping at a fence padlocked with a heavy chain. Behind it was a second armed man who patted down the producer and then directed him to a nearby table where a 20-something woman began asking a barrage of questions.

How did you find out about this place? Is it your first time? Are you a cop?

Once satisfied with the answers, she asked for an ID and $20. Then she directed the producer to a door in the rear of a nondescript building. He was not asked for his profession, and he didn’t volunteer the information.

The door opened to a rollicking scene inside. A brightly lit space with a DJ in the middle blasting hip-hop music and nearly a dozen tables filled with marijuana products in every form.

The event was blatantly illegal: an underground marijuana bazaar selling bulk amounts of counterfeit THC vape pens, edibles and cannabis flower—all of it unlicensed.

Public health officials in the United States are grappling with an epidemic of severe lung illnesses linked to vape pens, the majority involving bootleg THC oils. At least 60 people have died and nearly 2,700 others have been hospitalized, according to the Centers for Disease Control and Prevention.

The Food and Drug Administration and the Drug Enforcement Administration took a bold step in late December, taking down 44 websites that were advertising the sale of illicit THC cartridges.

But NBC News found that countless purveyors of the counterfeit products are hawking their wares in plain sight on social media pages, especially Instagram and Facebook. These illegal operators appear to be doing so with impunity, using the ease and anonymity of Instagram to reach a massive audience of young people who vape.

“They are facilitating a public health crisis,” said Timothy Mackey, a professor at the University of California San Diego, who has done extensive research on how social media platforms are used to sell illicit products.

“I think they should do something about it,” Mackey added, referring to Instagram. “This stuff is getting out there because these platforms are facilitating their sale.”

Shopping for illicit vape cartridges on Instagram is astonishingly simple. Open the app, plug in a hashtag such as #vapecartsforsale and — voilà — multiple posts appear with pictures of THC cartridges. In the comments or caption section, sellers advertise their products and post phone numbers for would-be buyers.

“Shop here everything is good,” one seller wrote above a phone number.

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“Everything must go,” another wrote in a separate post.

The CDC has identified a bootleg brand of THC cartridges called Dank Vapes as the source of several vape-related illnesses. NBC News has previously reported that the bogus brand’s cartridges contain contaminants such as hydrogen cyanide and vitamin E acetate, a cutting agent identified by the CDC as one of the likely culprits in the outbreak.

But until recently, you could type #dankvapesofficial into Instagram, and more than 49,000 posts would pop up. In one, a dealer uploaded a video of himself rummaging through a duffel bag that was overstuffed with Dank Vapes cartridges. In the caption, he posted his number, encouraging users to text him, using the encrypted messaging app, WhatsApp.

“Danks on danks on danks, baby!” he can be heard saying in the video.

In another Instagram post, showing three Dank Vape cartridges, a user listed a phone number and wrote in the comments, “hit me up if you need some!! Quit getting scammed.”

Experts say Instagram has become a go-to place for people promoting marijuana products, legal and illicit, because of its ease of use, huge popularity among young people and emphasis on photos and videos.

“With Instagram, a lot of these companies can go from zero to hero in 30 days,” said Avis Bulbulyan, CEO of Siva Enterprises, a cannabis consulting firm, and a member of California’s Cannabis Advisory Committee.

But the site is also rife with scammers.

Seeking to complete a sale, NBC News contacted about a dozen different dealers advertising Dank Vapes and ultimately transferred hundreds of dollars to three of them using a popular payment app. One of the dealers requested payment in the form of the electronic currency, Bitcoin.

But no drugs arrived — they were all scams. One of the dealers suggested meeting in a parking lot, but the person never showed.

Instagram’s official policy bars using the platform to advertise or sell any marijuana products, licensed or unlicensed.

In an interview, Karina Newton, Instagram’s head of public policy, emphasized that the company doesn’t want its platform to be used to sell cannabis products. “That’s not what Instagram is for,” she said.

Newton said Instagram uses special technology to help identify and remove vaping content banned from the platform. “That actually finds 95 percent of the content before it’s even reported to us,” she said.

Instagram and its parent company, Facebook, have taken steps in recent months to restrict the sale of certain products on its platform, including tobacco and e-cigarettes. But users with massive followings who are paid to promote products, known as influencers, were permitted to continue promoting vaping items.

Pressed on that decision, Newton said it is “something we definitely want to look at.”

Just a few days after the interview, Instagram announced it was banning branded content that promotes such goods as vaping and tobacco products.

Mackey, the professor, downplayed the significance of the move.

“This is not very effective as it only impacts certain content on the platform,” he said.

“This is a public health crisis of unknown magnitude,” he added. “Instagram should be more proactive about banning any offers for sale or promotion of any vaping products.”

The company did remove 19 of the 20 Facebook groups that NBC News found were advertising the sale of THC cartridges. It also took down nine of the 12 Instagram posts and two additional hashtags: #dankvapesofficial and #dankvapes420.

Mackey and other experts conceded that Instagram’s efforts to root out illicit sales are complicated by the methods taken by rogue sellers to elude detection.

The private account that posted the details of the underground marijuana bazaar in California listed the address not in an Instagram post but in a live story, which are far more difficult to police.

At the event, several vendors offered passing customers the chance to smell their marijuana flower. One seller was hawking vacuum-packed bags of buds the size of large backpacks for prices as high as $1,250. The NBC News producer witnessed one customer transfer $750 via CashApp to purchase a large quantity of marijuana.

Recreational marijuana is legal in California but only licensed dealers are permitted to sell it. Many of the sellers at the underground operation were peddling bootleg vape cartridges, including Dank Vapes.

One of the dealers offered the producer 30 THC cartridges for $300. The dealer turned down the producer's request to buy just five, saying he only sold in bulk.

The producer moved on to a different vendor and purchased a total of seven cartridges — five Dank Vapes and two KingPen, all counterfeits containing a potential toxic stew of chemicals — for $100.

Back outside, security would not allow him to leave the fenced-in premises until his taxi arrived. When it showed up, an armed guard led him to the vehicle, opened the door and even told him to watch his step getting in.

Then the car was off, his bootleg vape cartridges tucked into a black plastic bag — all thanks to Instagram.