Marijuana vape pens seized from illegal shops in Los Angeles contained potentially dangerous additives, including a substance linked to the outbreak of vaping-related lung illnesses, authorities said Monday.

The test results for some of the 10,000 confiscated vape cartridges serve as a reminder to avoid unlicensed marijuana retailers, California's Bureau of Cannabis Control said.

After seizing the products in raids last month, state officials determined 75% had undisclosed chemicals such as propylene glycol and vitamin E acetate, the thickening agent blamed for the national outbreak that has killed 60 people since last spring.

“The prevalence of dirty and dangerous vape pens at unlicensed cannabis stores demonstrate how important it is for consumers to purchase cannabis goods from licensed retailers, which are required to sell products that meet state testing and labeling standards,” Lori Ajax, chief of the Bureau of Cannabis Control, said in a statement.

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Confiscated products were also less potent than the labels claimed, with some containing less than half of the THC advertised. One cartridge tested by California public health officials had oil containing 33% THC, contradicting its label of 80 to 85% THC.

The report comes after the Center for Disease Control revealed about 77% of patients who developed lung injuries after vaping marijuana obtained the product from "informal" sources such as in-person or online dealers. About 16% bought it from commerican dispensaries, while the remainder said they got it from both, the CDC reported two weeks ago.

In addition to the 60 deaths, vaping has been linked to lung illness among more than 2,600 people across all 50 states. The CDC investigation began with no distinction between nicotine and marijuana-based THC. It grew to emphasize black market, street-sold THC cartridges and includes legally purchased products containing THC, the ingredient in marijuana that creates a high and is vaped in oil.

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To reduce underage vaping, the Food and Drug Administration said it will ban fruit- and mint-flavored products used in e-cigarettes. The administration will begin enforcing the rule by February and target companies marketing to youth.

Contributing: Jayne O'Donnell and Ken Alltucker, USA TODAY